diff --git a/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeGrid.js b/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeGrid.js
index 685ad33478205..3f0b9f40676b9 100644
--- a/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeGrid.js
+++ b/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeGrid.js
@@ -1,10 +1,12 @@
 // @flow
 
+import type {ComponentType} from "react";
+
 import createGridComponent from './createGridComponent';
 
 import type { Props, ScrollToAlign } from './createGridComponent';
 
-const FixedSizeGrid = createGridComponent({
+const FixedSizeGrid: ComponentType<Props<$FlowFixMe>> = createGridComponent({
   getColumnOffset: ({ columnWidth }: Props<any>, index: number): number =>
     index * ((columnWidth: any): number),
 
diff --git a/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeList.js b/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeList.js
index 0e0fc305bc301..5b6dab45988fb 100644
--- a/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeList.js
+++ b/packages/react-devtools-shared/src/node_modules/react-window/src/FixedSizeList.js
@@ -1,10 +1,12 @@
 // @flow
 
+import type {ComponentType} from "react";
+
 import createListComponent from './createListComponent';
 
 import type { Props, ScrollToAlign } from './createListComponent';
 
-const FixedSizeList = createListComponent({
+const FixedSizeList: ComponentType<Props<$FlowFixMe>> = createListComponent({
   getItemOffset: ({ itemSize }: Props<any>, index: number): number =>
     index * ((itemSize: any): number),
 
diff --git a/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeGrid.js b/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeGrid.js
index 2ce4f2d9a04ce..6b0d6a65e987a 100644
--- a/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeGrid.js
+++ b/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeGrid.js
@@ -1,5 +1,7 @@
 // @flow
 
+import type {ComponentType} from "react";
+
 import createGridComponent from './createGridComponent';
 
 import type { Props, ScrollToAlign } from './createGridComponent';
@@ -286,7 +288,7 @@ const getOffsetForIndexAndAlignment = (
   }
 };
 
-const VariableSizeGrid = createGridComponent({
+const VariableSizeGrid: ComponentType<Props<$FlowFixMe>> = createGridComponent({
   getColumnOffset: (
     props: Props<any>,
     index: number,
diff --git a/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeList.js b/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeList.js
index 887641b010eef..fa876ad994cbf 100644
--- a/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeList.js
+++ b/packages/react-devtools-shared/src/node_modules/react-window/src/VariableSizeList.js
@@ -1,5 +1,7 @@
 // @flow
 
+import type {ComponentType} from "react";
+
 import createListComponent from './createListComponent';
 
 import type { Props, ScrollToAlign } from './createListComponent';
@@ -163,7 +165,7 @@ const getEstimatedTotalSize = (
   return totalSizeOfMeasuredItems + totalSizeOfUnmeasuredItems;
 };
 
-const VariableSizeList = createListComponent({
+const VariableSizeList: ComponentType<Props<any>> = createListComponent({
   getItemOffset: (
     props: Props<any>,
     index: number,
diff --git a/packages/react-dom-bindings/src/client/ReactDOMInput.js b/packages/react-dom-bindings/src/client/ReactDOMInput.js
index 8d9ea1f45a431..ec03d75e37afb 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMInput.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMInput.js
@@ -59,7 +59,7 @@ function isControlled(props) {
  * See http://www.w3.org/TR/2012/WD-html5-20121025/the-input-element.html
  */
 
-export function getHostProps(element: Element, props: Object) {
+export function getHostProps(element: Element, props: Object): Object {
   const node = ((element: any): InputWithWrapperState);
   const checked = props.checked;
 
diff --git a/packages/react-dom-bindings/src/client/ReactDOMSelect.js b/packages/react-dom-bindings/src/client/ReactDOMSelect.js
index 57f53b5206a82..3493879cf7989 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMSelect.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMSelect.js
@@ -134,7 +134,7 @@ function updateOptions(
  * selected.
  */
 
-export function getHostProps(element: Element, props: Object) {
+export function getHostProps(element: Element, props: Object): Object {
   return assign({}, props, {
     value: undefined,
   });
diff --git a/packages/react-dom-bindings/src/client/ReactDOMTextarea.js b/packages/react-dom-bindings/src/client/ReactDOMTextarea.js
index 065f76d16c12d..4bf87f2e2340c 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMTextarea.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMTextarea.js
@@ -37,7 +37,7 @@ type TextAreaWithWrapperState = HTMLTextAreaElement & {
  * `defaultValue` if specified, or the children content (deprecated).
  */
 
-export function getHostProps(element: Element, props: Object) {
+export function getHostProps(element: Element, props: Object): Object {
   const node = ((element: any): TextAreaWithWrapperState);
 
   if (props.dangerouslySetInnerHTML != null) {
diff --git a/packages/react-dom-bindings/src/client/setInnerHTML.js b/packages/react-dom-bindings/src/client/setInnerHTML.js
index e283efb270770..fd4eb00748e95 100644
--- a/packages/react-dom-bindings/src/client/setInnerHTML.js
+++ b/packages/react-dom-bindings/src/client/setInnerHTML.js
@@ -21,7 +21,10 @@ let reusableSVGContainer;
  * @param {string} html
  * @internal
  */
-const setInnerHTML = createMicrosoftUnsafeLocalFunction(function(
+const setInnerHTML: (
+  node: Element,
+  html: {valueOf(): {toString(): string, ...}, ...},
+) => void = createMicrosoftUnsafeLocalFunction(function(
   node: Element,
   html: {valueOf(): {toString(): string, ...}, ...},
 ): void {
diff --git a/packages/react-dom-bindings/src/events/EventRegistry.js b/packages/react-dom-bindings/src/events/EventRegistry.js
index eed8b9dc49845..f86948f8d5227 100644
--- a/packages/react-dom-bindings/src/events/EventRegistry.js
+++ b/packages/react-dom-bindings/src/events/EventRegistry.js
@@ -21,7 +21,9 @@ if (enableCreateEventHandleAPI) {
 /**
  * Mapping from registration name to event name
  */
-export const registrationNameDependencies = {};
+export const registrationNameDependencies: {
+  [registrationName: string]: Array<DOMEventName>,
+} = {};
 
 /**
  * Mapping from lowercase registration names to the properly cased version,
@@ -29,7 +31,9 @@ export const registrationNameDependencies = {};
  * only in __DEV__.
  * @type {Object}
  */
-export const possibleRegistrationNames = __DEV__ ? {} : (null: any);
+export const possibleRegistrationNames: {
+  [lowerCasedName: string]: string,
+} = __DEV__ ? {} : (null: any);
 // Trust the developer to only use possibleRegistrationNames in __DEV__
 
 export function registerTwoPhaseEvent(
diff --git a/packages/react-dom-bindings/src/events/ReactDOMEventListener.js b/packages/react-dom-bindings/src/events/ReactDOMEventListener.js
index bd7e05b1d760e..ba2c1e63239de 100644
--- a/packages/react-dom-bindings/src/events/ReactDOMEventListener.js
+++ b/packages/react-dom-bindings/src/events/ReactDOMEventListener.js
@@ -8,7 +8,7 @@
  */
 
 import type {AnyNativeEvent} from '../events/PluginModuleType';
-import type {FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
+import type {Fiber, FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
 import type {Container, SuspenseInstance} from '../client/ReactDOMHostConfig';
 import type {DOMEventName} from '../events/DOMEventNames';
 import {enableCapturePhaseSelectiveHydrationWithoutDiscreteEventReplay} from 'shared/ReactFeatureFlags';
@@ -58,15 +58,15 @@ import {isRootDehydrated} from 'react-reconciler/src/ReactFiberShellHydration';
 const {ReactCurrentBatchConfig} = ReactSharedInternals;
 
 // TODO: can we stop exporting these?
-export let _enabled = true;
+export let _enabled: boolean = true;
 
 // This is exported in FB builds for use by legacy FB layer infra.
 // We'd like to remove this but it's not clear if this is safe.
-export function setEnabled(enabled: ?boolean) {
+export function setEnabled(enabled: ?boolean): void {
   _enabled = !!enabled;
 }
 
-export function isEnabled() {
+export function isEnabled(): boolean {
   return _enabled;
 }
 
@@ -348,7 +348,7 @@ function dispatchEventWithEnableCapturePhaseSelectiveHydrationWithoutDiscreteEve
   );
 }
 
-export let return_targetInst = null;
+export let return_targetInst: null | Fiber = null;
 
 // Returns a SuspenseInstance or Container if it's blocked.
 // The return_targetInst field above is conceptually part of the return value.
diff --git a/packages/react-dom-bindings/src/events/SyntheticEvent.js b/packages/react-dom-bindings/src/events/SyntheticEvent.js
index 87bdad3548cdc..2d75b647aca80 100644
--- a/packages/react-dom-bindings/src/events/SyntheticEvent.js
+++ b/packages/react-dom-bindings/src/events/SyntheticEvent.js
@@ -16,7 +16,6 @@ import getEventCharCode from './getEventCharCode';
 
 type EventInterfaceType = {
   [propName: string]: 0 | ((event: {[propName: string]: mixed, ...}) => mixed),
-  ...,
 };
 
 function functionThatReturnsTrue() {
@@ -153,14 +152,16 @@ const EventInterface = {
   defaultPrevented: 0,
   isTrusted: 0,
 };
-export const SyntheticEvent = createSyntheticEvent(EventInterface);
+export const SyntheticEvent: $FlowFixMe = createSyntheticEvent(EventInterface);
 
 const UIEventInterface: EventInterfaceType = {
   ...EventInterface,
   view: 0,
   detail: 0,
 };
-export const SyntheticUIEvent = createSyntheticEvent(UIEventInterface);
+export const SyntheticUIEvent: $FlowFixMe = createSyntheticEvent(
+  UIEventInterface,
+);
 
 let lastMovementX;
 let lastMovementY;
@@ -225,7 +226,9 @@ const MouseEventInterface: EventInterfaceType = {
     return lastMovementY;
   },
 };
-export const SyntheticMouseEvent = createSyntheticEvent(MouseEventInterface);
+export const SyntheticMouseEvent: $FlowFixMe = createSyntheticEvent(
+  MouseEventInterface,
+);
 
 /**
  * @interface DragEvent
@@ -235,7 +238,9 @@ const DragEventInterface: EventInterfaceType = {
   ...MouseEventInterface,
   dataTransfer: 0,
 };
-export const SyntheticDragEvent = createSyntheticEvent(DragEventInterface);
+export const SyntheticDragEvent: $FlowFixMe = createSyntheticEvent(
+  DragEventInterface,
+);
 
 /**
  * @interface FocusEvent
@@ -245,7 +250,9 @@ const FocusEventInterface: EventInterfaceType = {
   ...UIEventInterface,
   relatedTarget: 0,
 };
-export const SyntheticFocusEvent = createSyntheticEvent(FocusEventInterface);
+export const SyntheticFocusEvent: $FlowFixMe = createSyntheticEvent(
+  FocusEventInterface,
+);
 
 /**
  * @interface Event
@@ -258,7 +265,7 @@ const AnimationEventInterface: EventInterfaceType = {
   elapsedTime: 0,
   pseudoElement: 0,
 };
-export const SyntheticAnimationEvent = createSyntheticEvent(
+export const SyntheticAnimationEvent: $FlowFixMe = createSyntheticEvent(
   AnimationEventInterface,
 );
 
@@ -274,7 +281,7 @@ const ClipboardEventInterface: EventInterfaceType = {
       : window.clipboardData;
   },
 };
-export const SyntheticClipboardEvent = createSyntheticEvent(
+export const SyntheticClipboardEvent: $FlowFixMe = createSyntheticEvent(
   ClipboardEventInterface,
 );
 
@@ -286,7 +293,7 @@ const CompositionEventInterface: EventInterfaceType = {
   ...EventInterface,
   data: 0,
 };
-export const SyntheticCompositionEvent = createSyntheticEvent(
+export const SyntheticCompositionEvent: $FlowFixMe = createSyntheticEvent(
   CompositionEventInterface,
 );
 
@@ -487,7 +494,7 @@ const KeyboardEventInterface = {
     return 0;
   },
 };
-export const SyntheticKeyboardEvent = createSyntheticEvent(
+export const SyntheticKeyboardEvent: $FlowFixMe = createSyntheticEvent(
   KeyboardEventInterface,
 );
 
@@ -508,7 +515,7 @@ const PointerEventInterface = {
   pointerType: 0,
   isPrimary: 0,
 };
-export const SyntheticPointerEvent = createSyntheticEvent(
+export const SyntheticPointerEvent: $FlowFixMe = createSyntheticEvent(
   PointerEventInterface,
 );
 
@@ -527,7 +534,9 @@ const TouchEventInterface = {
   shiftKey: 0,
   getModifierState: getEventModifierState,
 };
-export const SyntheticTouchEvent = createSyntheticEvent(TouchEventInterface);
+export const SyntheticTouchEvent: $FlowFixMe = createSyntheticEvent(
+  TouchEventInterface,
+);
 
 /**
  * @interface Event
@@ -540,7 +549,7 @@ const TransitionEventInterface = {
   elapsedTime: 0,
   pseudoElement: 0,
 };
-export const SyntheticTransitionEvent = createSyntheticEvent(
+export const SyntheticTransitionEvent: $FlowFixMe = createSyntheticEvent(
   TransitionEventInterface,
 );
 
@@ -580,4 +589,6 @@ const WheelEventInterface = {
   // ~40 pixels, for DOM_DELTA_SCREEN (2) it is 87.5% of viewport size.
   deltaMode: 0,
 };
-export const SyntheticWheelEvent = createSyntheticEvent(WheelEventInterface);
+export const SyntheticWheelEvent: $FlowFixMe = createSyntheticEvent(
+  WheelEventInterface,
+);
diff --git a/packages/react-dom-bindings/src/events/forks/EventListener-www.js b/packages/react-dom-bindings/src/events/forks/EventListener-www.js
index 04521613b5962..b9f20cca800ad 100644
--- a/packages/react-dom-bindings/src/events/forks/EventListener-www.js
+++ b/packages/react-dom-bindings/src/events/forks/EventListener-www.js
@@ -16,7 +16,7 @@ export function addEventBubbleListener(
   target: EventTarget,
   eventType: string,
   listener: Function,
-) {
+): mixed {
   return EventListenerWWW.listen(target, eventType, listener);
 }
 
@@ -24,7 +24,7 @@ export function addEventCaptureListener(
   target: EventTarget,
   eventType: string,
   listener: Function,
-) {
+): mixed {
   return EventListenerWWW.capture(target, eventType, listener);
 }
 
@@ -33,7 +33,7 @@ export function addEventCaptureListenerWithPassiveFlag(
   eventType: string,
   listener: Function,
   passive: boolean,
-) {
+): mixed {
   return EventListenerWWW.captureWithPassiveFlag(
     target,
     eventType,
@@ -47,7 +47,7 @@ export function addEventBubbleListenerWithPassiveFlag(
   eventType: string,
   listener: Function,
   passive: boolean,
-) {
+): mixed {
   return EventListenerWWW.bubbleWithPassiveFlag(
     target,
     eventType,
diff --git a/packages/react-dom/src/client/ReactDOM.js b/packages/react-dom/src/client/ReactDOM.js
index 849bc283e01db..3f3265043e881 100644
--- a/packages/react-dom/src/client/ReactDOM.js
+++ b/packages/react-dom/src/client/ReactDOM.js
@@ -8,7 +8,10 @@
  */
 
 import type {ReactNodeList} from 'shared/ReactTypes';
-import type {Container} from 'react-dom-bindings/src/client/ReactDOMHostConfig';
+import type {
+  Container,
+  PublicInstance,
+} from 'react-dom-bindings/src/client/ReactDOMHostConfig';
 import type {
   RootType,
   HydrateRootOptions,
@@ -125,7 +128,7 @@ function renderSubtreeIntoContainer(
   element: React$Element<any>,
   containerNode: Container,
   callback: ?Function,
-) {
+): React$Component<any, any> | PublicInstance | null {
   return unstable_renderSubtreeIntoContainer(
     parentComponent,
     element,
@@ -171,7 +174,7 @@ declare function flushSync<R>(fn: () => R): R;
 // eslint-disable-next-line no-redeclare
 declare function flushSync(): void;
 // eslint-disable-next-line no-redeclare
-function flushSync(fn) {
+function flushSync<R>(fn: (() => R) | void): R | void {
   if (__DEV__) {
     if (isAlreadyRendering()) {
       console.error(
diff --git a/packages/react-dom/src/client/ReactDOMLegacy.js b/packages/react-dom/src/client/ReactDOMLegacy.js
index 7118ba1e871c7..92f1ff0cdca5f 100644
--- a/packages/react-dom/src/client/ReactDOMLegacy.js
+++ b/packages/react-dom/src/client/ReactDOMLegacy.js
@@ -7,7 +7,10 @@
  * @flow
  */
 
-import type {Container} from 'react-dom-bindings/src/client/ReactDOMHostConfig';
+import type {
+  Container,
+  PublicInstance,
+} from 'react-dom-bindings/src/client/ReactDOMHostConfig';
 import type {FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
 import type {ReactNodeList} from 'shared/ReactTypes';
 
@@ -206,7 +209,7 @@ function legacyRenderSubtreeIntoContainer(
   container: Container,
   forceHydrate: boolean,
   callback: ?Function,
-) {
+): React$Component<any, any> | PublicInstance | null {
   if (__DEV__) {
     topLevelUpdateWarnings(container);
     warnOnInvalidCallback(callback === undefined ? null : callback, 'render');
@@ -274,7 +277,7 @@ export function hydrate(
   element: React$Node,
   container: Container,
   callback: ?Function,
-) {
+): React$Component<any, any> | PublicInstance | null {
   if (__DEV__) {
     console.error(
       'ReactDOM.hydrate is no longer supported in React 18. Use hydrateRoot ' +
@@ -314,7 +317,7 @@ export function render(
   element: React$Element<any>,
   container: Container,
   callback: ?Function,
-) {
+): React$Component<any, any> | PublicInstance | null {
   if (__DEV__) {
     console.error(
       'ReactDOM.render is no longer supported in React 18. Use createRoot ' +
@@ -354,7 +357,7 @@ export function unstable_renderSubtreeIntoContainer(
   element: React$Element<any>,
   containerNode: Container,
   callback: ?Function,
-) {
+): React$Component<any, any> | PublicInstance | null {
   if (__DEV__) {
     console.error(
       'ReactDOM.unstable_renderSubtreeIntoContainer() is no longer supported ' +
@@ -381,7 +384,7 @@ export function unstable_renderSubtreeIntoContainer(
   );
 }
 
-export function unmountComponentAtNode(container: Container) {
+export function unmountComponentAtNode(container: Container): boolean {
   if (!isValidContainerLegacy(container)) {
     throw new Error(
       'unmountComponentAtNode(...): Target container is not a DOM element.',
diff --git a/packages/react-native-renderer/src/ReactFabricHostConfig.js b/packages/react-native-renderer/src/ReactFabricHostConfig.js
index d78339b8128ea..4284889658c5c 100644
--- a/packages/react-native-renderer/src/ReactFabricHostConfig.js
+++ b/packages/react-native-renderer/src/ReactFabricHostConfig.js
@@ -593,7 +593,7 @@ export function replaceContainerChildren(
   newChildren: ChildSet,
 ): void {}
 
-export function getInstanceFromNode(node: any) {
+export function getInstanceFromNode(node: any): empty {
   throw new Error('Not yet implemented.');
 }
 
diff --git a/packages/react-native-renderer/src/ReactNativeHostConfig.js b/packages/react-native-renderer/src/ReactNativeHostConfig.js
index 5deae0672e6ba..26855bd87e838 100644
--- a/packages/react-native-renderer/src/ReactNativeHostConfig.js
+++ b/packages/react-native-renderer/src/ReactNativeHostConfig.js
@@ -492,7 +492,7 @@ export function unhideTextInstance(
   throw new Error('Not yet implemented.');
 }
 
-export function getInstanceFromNode(node: any) {
+export function getInstanceFromNode(node: any): empty {
   throw new Error('Not yet implemented.');
 }
 
diff --git a/packages/react-native-renderer/src/legacy-events/accumulate.js b/packages/react-native-renderer/src/legacy-events/accumulate.js
index 6d43ada04f0ef..9d4fe02b4793c 100644
--- a/packages/react-native-renderer/src/legacy-events/accumulate.js
+++ b/packages/react-native-renderer/src/legacy-events/accumulate.js
@@ -39,6 +39,8 @@ function accumulate<T>(
   }
 
   if (isArray(next)) {
+    /* $FlowFixMe[incompatible-return] unsound if `next` is `T` and `T` an array,
+     * `isArray` might refine to the array element type of `T` */
     return [current].concat(next);
   }
 
diff --git a/packages/react-native-renderer/src/legacy-events/accumulateInto.js b/packages/react-native-renderer/src/legacy-events/accumulateInto.js
index 7c9f17a757793..6016a1383dab3 100644
--- a/packages/react-native-renderer/src/legacy-events/accumulateInto.js
+++ b/packages/react-native-renderer/src/legacy-events/accumulateInto.js
@@ -51,6 +51,8 @@ function accumulateInto<T>(
 
   if (isArray(next)) {
     // A bit too dangerous to mutate `next`.
+    /* $FlowFixMe[incompatible-return] unsound if `next` is `T` and `T` an array,
+     * `isArray` might refine to the array element type of `T` */
     return [current].concat(next);
   }
 
diff --git a/packages/react-reconciler/src/ReactFiberHostConfigWithNoHydration.js b/packages/react-reconciler/src/ReactFiberHostConfigWithNoHydration.js
index f61c0eb7789e7..72d2c72fe2814 100644
--- a/packages/react-reconciler/src/ReactFiberHostConfigWithNoHydration.js
+++ b/packages/react-reconciler/src/ReactFiberHostConfigWithNoHydration.js
@@ -10,7 +10,7 @@
 // Renderers that don't support hydration
 // can re-export everything from this module.
 
-function shim(...args: any) {
+function shim(...args: any): empty {
   throw new Error(
     'The current renderer does not support hydration. ' +
       'This error is likely caused by a bug in React. ' +
diff --git a/packages/react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks.js b/packages/react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks.js
index 5c55d9d59b01f..f50545747cffe 100644
--- a/packages/react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks.js
+++ b/packages/react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks.js
@@ -10,7 +10,7 @@
 // Renderers that don't support microtasks
 // can re-export everything from this module.
 
-function shim(...args: any) {
+function shim(...args: any): empty {
   throw new Error(
     'The current renderer does not support microtasks. ' +
       'This error is likely caused by a bug in React. ' +
diff --git a/packages/react-reconciler/src/ReactFiberHostConfigWithNoMutation.js b/packages/react-reconciler/src/ReactFiberHostConfigWithNoMutation.js
index 59e8c450861f0..742e115f13334 100644
--- a/packages/react-reconciler/src/ReactFiberHostConfigWithNoMutation.js
+++ b/packages/react-reconciler/src/ReactFiberHostConfigWithNoMutation.js
@@ -10,7 +10,7 @@
 // Renderers that don't support mutation
 // can re-export everything from this module.
 
-function shim(...args: any) {
+function shim(...args: any): empty {
   throw new Error(
     'The current renderer does not support mutation. ' +
       'This error is likely caused by a bug in React. ' +
diff --git a/packages/react-reconciler/src/ReactFiberHostConfigWithNoPersistence.js b/packages/react-reconciler/src/ReactFiberHostConfigWithNoPersistence.js
index c34449f6a2466..85d0d83caa533 100644
--- a/packages/react-reconciler/src/ReactFiberHostConfigWithNoPersistence.js
+++ b/packages/react-reconciler/src/ReactFiberHostConfigWithNoPersistence.js
@@ -10,7 +10,7 @@
 // Renderers that don't support persistence
 // can re-export everything from this module.
 
-function shim(...args: any) {
+function shim(...args: any): empty {
   throw new Error(
     'The current renderer does not support persistence. ' +
       'This error is likely caused by a bug in React. ' +
diff --git a/packages/react-reconciler/src/ReactFiberHostConfigWithNoResources.js b/packages/react-reconciler/src/ReactFiberHostConfigWithNoResources.js
index b620a06b37fb9..1075834cb9bd0 100644
--- a/packages/react-reconciler/src/ReactFiberHostConfigWithNoResources.js
+++ b/packages/react-reconciler/src/ReactFiberHostConfigWithNoResources.js
@@ -10,7 +10,7 @@
 // Renderers that don't support hydration
 // can re-export everything from this module.
 
-function shim(...args: any) {
+function shim(...args: any): empty {
   throw new Error(
     'The current renderer does not support Resources. ' +
       'This error is likely caused by a bug in React. ' +
diff --git a/packages/react-reconciler/src/ReactFiberHostConfigWithNoScopes.js b/packages/react-reconciler/src/ReactFiberHostConfigWithNoScopes.js
index 6eb3d08a4de64..3d4064ebfdb5f 100644
--- a/packages/react-reconciler/src/ReactFiberHostConfigWithNoScopes.js
+++ b/packages/react-reconciler/src/ReactFiberHostConfigWithNoScopes.js
@@ -10,7 +10,7 @@
 // Renderers that don't support React Scopes
 // can re-export everything from this module.
 
-function shim(...args: any) {
+function shim(...args: any): empty {
   throw new Error(
     'The current renderer does not support React Scopes. ' +
       'This error is likely caused by a bug in React. ' +
diff --git a/packages/react-reconciler/src/ReactFiberHostConfigWithNoTestSelectors.js b/packages/react-reconciler/src/ReactFiberHostConfigWithNoTestSelectors.js
index 964bf4b2c43d5..a597ca526565d 100644
--- a/packages/react-reconciler/src/ReactFiberHostConfigWithNoTestSelectors.js
+++ b/packages/react-reconciler/src/ReactFiberHostConfigWithNoTestSelectors.js
@@ -10,7 +10,7 @@
 // Renderers that don't support test selectors
 // can re-export everything from this module.
 
-function shim(...args: any) {
+function shim(...args: any): empty {
   throw new Error(
     'The current renderer does not support test selectors. ' +
       'This error is likely caused by a bug in React. ' +
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index e0add4c8db6fd..34b9d3f10c06b 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -1565,7 +1565,7 @@ export function discreteUpdates<A, B, C, D, R>(
 // Warning, this opts-out of checking the function body.
 declare function flushSync<R>(fn: () => R): R;
 // eslint-disable-next-line no-redeclare
-declare function flushSync(): void;
+declare function flushSync(void): void;
 // eslint-disable-next-line no-redeclare
 export function flushSync<R>(fn: (() => R) | void): R | void {
   // In legacy mode, we flush pending passive effects at the beginning of the
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index e5774487dc2cf..b5cd4565ef104 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -1565,7 +1565,7 @@ export function discreteUpdates<A, B, C, D, R>(
 // Warning, this opts-out of checking the function body.
 declare function flushSync<R>(fn: () => R): R;
 // eslint-disable-next-line no-redeclare
-declare function flushSync(): void;
+declare function flushSync(void): void;
 // eslint-disable-next-line no-redeclare
 export function flushSync<R>(fn: (() => R) | void): R | void {
   // In legacy mode, we flush pending passive effects at the beginning of the
diff --git a/packages/react-test-renderer/src/ReactTestHostConfig.js b/packages/react-test-renderer/src/ReactTestHostConfig.js
index 1db038e411286..e8daa8368f9e7 100644
--- a/packages/react-test-renderer/src/ReactTestHostConfig.js
+++ b/packages/react-test-renderer/src/ReactTestHostConfig.js
@@ -57,7 +57,7 @@ if (__DEV__) {
   Object.freeze(UPDATE_SIGNAL);
 }
 
-export function getPublicInstance(inst: Instance | TextInstance): * {
+export function getPublicInstance(inst: Instance | TextInstance): $FlowFixMe {
   switch (inst.tag) {
     case 'INSTANCE':
       const createNodeMock = inst.rootContainerInstance.createNodeMock;
@@ -283,7 +283,7 @@ export function unhideTextInstance(
   textInstance.isHidden = false;
 }
 
-export function getInstanceFromNode(mockNode: Object) {
+export function getInstanceFromNode(mockNode: Object): Object | null {
   const instance = nodeToInstanceMap.get(mockNode);
   if (instance !== undefined) {
     return instance.internalInstanceHandle;
diff --git a/packages/react-test-renderer/src/ReactTestRenderer.js b/packages/react-test-renderer/src/ReactTestRenderer.js
index f8d8b8ca3a37a..f0ba1fce9c0b6 100644
--- a/packages/react-test-renderer/src/ReactTestRenderer.js
+++ b/packages/react-test-renderer/src/ReactTestRenderer.js
@@ -9,7 +9,11 @@
 
 import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
 import type {FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
-import type {Instance, TextInstance} from './ReactTestHostConfig';
+import type {
+  PublicInstance,
+  Instance,
+  TextInstance,
+} from './ReactTestHostConfig';
 
 import * as React from 'react';
 import * as Scheduler from 'scheduler/unstable_mock';
@@ -303,7 +307,7 @@ class ReactTestInstance {
     this._fiber = fiber;
   }
 
-  get instance() {
+  get instance(): $FlowFixMe {
     if (this._fiber.tag === HostComponent || this._fiber.tag === HostResource) {
       return getPublicInstance(this._fiber.stateNode);
     } else {
@@ -311,7 +315,7 @@ class ReactTestInstance {
     }
   }
 
-  get type() {
+  get type(): any {
     return this._fiber.type;
   }
 
@@ -445,7 +449,19 @@ function onRecoverableError(error) {
   console.error(error);
 }
 
-function create(element: React$Element<any>, options: TestRendererOptions) {
+function create(
+  element: React$Element<any>,
+  options: TestRendererOptions,
+): {
+  _Scheduler: typeof Scheduler,
+  root: void,
+  toJSON(): Array<ReactTestRendererNode> | ReactTestRendererNode | null,
+  toTree(): mixed,
+  update(newElement: React$Element<any>): any,
+  unmount(): void,
+  getInstance(): React$Component<any, any> | PublicInstance | null,
+  unstable_flushSync: typeof flushSync,
+} {
   let createNodeMock = defaultTestOptions.createNodeMock;
   let isConcurrent = false;
   let isStrictMode = false;
@@ -534,7 +550,7 @@ function create(element: React$Element<any>, options: TestRendererOptions) {
       }
       return toTree(root.current);
     },
-    update(newElement: React$Element<any>) {
+    update(newElement: React$Element<any>): number | void {
       if (root == null || root.current == null) {
         return;
       }
diff --git a/packages/react/src/React.js b/packages/react/src/React.js
index fc6c77cbb6539..5edcd9e83049b 100644
--- a/packages/react/src/React.js
+++ b/packages/react/src/React.js
@@ -71,9 +71,15 @@ import {startTransition} from './ReactStartTransition';
 import {act} from './ReactAct';
 
 // TODO: Move this branching into the other module instead and just re-export.
-const createElement = __DEV__ ? createElementWithValidation : createElementProd;
-const cloneElement = __DEV__ ? cloneElementWithValidation : cloneElementProd;
-const createFactory = __DEV__ ? createFactoryWithValidation : createFactoryProd;
+const createElement: any = __DEV__
+  ? createElementWithValidation
+  : createElementProd;
+const cloneElement: any = __DEV__
+  ? cloneElementWithValidation
+  : cloneElementProd;
+const createFactory: any = __DEV__
+  ? createFactoryWithValidation
+  : createFactoryProd;
 
 const Children = {
   map,
diff --git a/packages/react/src/ReactDebugCurrentFrame.js b/packages/react/src/ReactDebugCurrentFrame.js
index ddaeed70f99b7..557a4cd56abc6 100644
--- a/packages/react/src/ReactDebugCurrentFrame.js
+++ b/packages/react/src/ReactDebugCurrentFrame.js
@@ -7,11 +7,17 @@
  * @flow
  */
 
-const ReactDebugCurrentFrame = {};
+const ReactDebugCurrentFrame: {
+  setExtraStackFrame?: (stack: null | string) => void,
+  getCurrentStack?: null | (() => string),
+  getStackAddendum?: () => string,
+} =
+  // $FlowFixMe[incompatible-exact]
+  {};
 
 let currentExtraStackFrame = (null: null | string);
 
-export function setExtraStackFrame(stack: null | string) {
+export function setExtraStackFrame(stack: null | string): void {
   if (__DEV__) {
     currentExtraStackFrame = stack;
   }
diff --git a/packages/react/src/jsx/ReactJSX.js b/packages/react/src/jsx/ReactJSX.js
index a4e8f63dedd74..f0134421ec51f 100644
--- a/packages/react/src/jsx/ReactJSX.js
+++ b/packages/react/src/jsx/ReactJSX.js
@@ -13,10 +13,10 @@ import {
   jsxWithValidation,
 } from './ReactJSXElementValidator';
 import {jsx as jsxProd} from './ReactJSXElement';
-const jsx = __DEV__ ? jsxWithValidationDynamic : jsxProd;
+const jsx: any = __DEV__ ? jsxWithValidationDynamic : jsxProd;
 // we may want to special case jsxs internally to take advantage of static children.
 // for now we can ship identical prod functions
-const jsxs = __DEV__ ? jsxWithValidationStatic : jsxProd;
-const jsxDEV = __DEV__ ? jsxWithValidation : undefined;
+const jsxs: any = __DEV__ ? jsxWithValidationStatic : jsxProd;
+const jsxDEV: any = __DEV__ ? jsxWithValidation : undefined;
 
 export {REACT_FRAGMENT_TYPE as Fragment, jsx, jsxs, jsxDEV};
diff --git a/scripts/flow/config/flowconfig b/scripts/flow/config/flowconfig
index 68e1aef4a3408..0299da6337508 100644
--- a/scripts/flow/config/flowconfig
+++ b/scripts/flow/config/flowconfig
@@ -45,46 +45,8 @@ esproposal.class_instance_fields=enable
 esproposal.optional_chaining=enable
 exact_by_default=true
 munge_underscores=false
-types_first=false
-
+types_first=true
 well_formed_exports=true
-well_formed_exports.includes=<PROJECT_ROOT>/packages/dom-event-testing-library
-well_formed_exports.includes=<PROJECT_ROOT>/packages/eslint-plugin-react-hooks
-well_formed_exports.includes=<PROJECT_ROOT>/packages/jest-mock-scheduler
-well_formed_exports.includes=<PROJECT_ROOT>/packages/jest-react
-; well_formed_exports.includes=<PROJECT_ROOT>/packages/react
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-art
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-cache
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-client
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-debug-tools
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-devtools
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-devtools-core
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-devtools-extensions
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-devtools-inline
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-devtools-shared
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-devtools-shell
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-devtools-timeline
-; well_formed_exports.includes=<PROJECT_ROOT>/packages/react-dom
-; well_formed_exports.includes=<PROJECT_ROOT>/packages/react-dom-bindings
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-fetch
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-fs
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-interactions
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-is
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-native-renderer
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-noop-renderer
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-pg
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-reconciler
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-refresh
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-server
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-server-dom-relay
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-server-dom-webpack
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-server-native-relay
-well_formed_exports.includes=<PROJECT_ROOT>/packages/react-suspense-test-utils
-; well_formed_exports.includes=<PROJECT_ROOT>/packages/react-test-renderer
-well_formed_exports.includes=<PROJECT_ROOT>/packages/scheduler
-well_formed_exports.includes=<PROJECT_ROOT>/packages/shared
-well_formed_exports.includes=<PROJECT_ROOT>/packages/use-subscription
-well_formed_exports.includes=<PROJECT_ROOT>/packages/use-sync-external-store
 
 # Substituted by createFlowConfig.js:
 %REACT_RENDERER_FLOW_OPTIONS%