diff --git a/.eslintrc.json b/.eslintrc.json
index 52793e7c..80d4ed74 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -13,6 +13,7 @@
   },
   "reportUnusedDisableDirectives": true,
   "rules": {
+    "@typescript-eslint/explicit-function-return-type": "error",
     "max-lines": ["warn", 500]
   }
 }
diff --git a/lib/__tests__/cookieJar.spec.ts b/lib/__tests__/cookieJar.spec.ts
index 03b1648a..208698e9 100644
--- a/lib/__tests__/cookieJar.spec.ts
+++ b/lib/__tests__/cookieJar.spec.ts
@@ -741,7 +741,7 @@ describe('CookieJar', () => {
       beforeEach(async () => {
         const url = 'http://example.com/index.html'
 
-        const at = (timeFromNow: number) => ({
+        const at = (timeFromNow: number): { now: Date } => ({
           now: new Date(Date.now() + timeFromNow),
         })
 
@@ -838,7 +838,7 @@ describe('CookieJar', () => {
       beforeEach(async () => {
         const url = 'http://example.com/index.html'
 
-        const at = (timeFromNow: number) => ({
+        const at = (timeFromNow: number): { now: Date } => ({
           now: new Date(Date.now() + timeFromNow),
         })
 
@@ -1506,7 +1506,7 @@ function apiVariants(
   testName: string,
   apiVariants: ApiVariants,
   assertions: () => void,
-) {
+): void {
   it(`${testName} (callback)`, async () => {
     await new Promise((resolve) =>
       apiVariants.callbackStyle(() => resolve(undefined)),
diff --git a/lib/__tests__/jarSerialization.spec.ts b/lib/__tests__/jarSerialization.spec.ts
index cf76f6e1..e189d0e6 100644
--- a/lib/__tests__/jarSerialization.spec.ts
+++ b/lib/__tests__/jarSerialization.spec.ts
@@ -299,7 +299,7 @@ describe('cookieJar serialization', () => {
 
 function expectDataToMatchSerializationSchema(
   serializedJar: SerializedCookieJar,
-) {
+): void {
   expect(serializedJar).not.toBeNull()
   expect(serializedJar).toBeInstanceOf(Object)
   expect(serializedJar.version).toBe(`tough-cookie@${version}`)
@@ -326,7 +326,7 @@ const serializedCookiePropTypes: { [key: string]: string } = {
   sameSite: 'string',
 }
 
-function validateSerializedCookie(cookie: SerializedCookie) {
+function validateSerializedCookie(cookie: SerializedCookie): void {
   expect(typeof cookie).toBe('object')
   expect(cookie).not.toBeInstanceOf(Cookie)
 
@@ -362,7 +362,7 @@ function validateSerializedCookie(cookie: SerializedCookie) {
   })
 }
 
-function isInteger(value: unknown) {
+function isInteger(value: unknown): boolean {
   if (Number.isInteger) {
     return Number.isInteger(value)
   }
diff --git a/lib/__tests__/regression.spec.ts b/lib/__tests__/regression.spec.ts
index daf26a2e..62d0dccc 100644
--- a/lib/__tests__/regression.spec.ts
+++ b/lib/__tests__/regression.spec.ts
@@ -36,7 +36,7 @@ describe('Regression Tests', () => {
     expect.assertions(2)
     const cookieJar = new CookieJar()
 
-    const callback = function (err: null, cookie: Cookie) {
+    const callback = function (err: null, cookie: Cookie): void {
       expect(err).toBeNull()
       expect(cookie).toEqual(
         expect.objectContaining({
@@ -58,7 +58,7 @@ describe('Regression Tests', () => {
     expect.assertions(2)
     const cookieJar = new CookieJar()
 
-    const callback = function (err: null, cookie: Cookie) {
+    const callback = function (err: null, cookie: Cookie): void {
       expect(err).toBeNull()
       expect(cookie).toEqual([
         expect.objectContaining({
diff --git a/lib/__tests__/util.spec.ts b/lib/__tests__/util.spec.ts
index afcda6c2..a16bf063 100644
--- a/lib/__tests__/util.spec.ts
+++ b/lib/__tests__/util.spec.ts
@@ -11,7 +11,7 @@ describe('safeToString', () => {
     [123, '123'],
     [321n, '321'],
     [{ object: 'yes' }, '[object Object]'],
-    [(a: number, b: number) => a + b, '(a, b) => a + b'],
+    [(a: number, b: number): number => a + b, '(a, b) => a + b'],
     [Symbol('safeToString'), 'Symbol(safeToString)'],
     [Object.create(null), '[object Object]'],
     // eslint-disable-next-line no-sparse-arrays
diff --git a/lib/cookie/cookie.ts b/lib/cookie/cookie.ts
index 2d386a10..f3e07cfb 100644
--- a/lib/cookie/cookie.ts
+++ b/lib/cookie/cookie.ts
@@ -57,7 +57,7 @@ const CONTROL_CHARS = /[\x00-\x1F]/
 // https://github.com/ChromiumWebApps/chromium/blob/b3d3b4da8bb94c1b2e061600df106d590fda3620/net/cookies/parsed_cookie.cc#L60
 const TERMINATORS = ['\n', '\r', '\0']
 
-function trimTerminator(str: string) {
+function trimTerminator(str: string): string {
   if (validators.isEmptyString(str)) return str
   for (let t = 0; t < TERMINATORS.length; t++) {
     const terminator = TERMINATORS[t]
@@ -70,7 +70,10 @@ function trimTerminator(str: string) {
   return str
 }
 
-function parseCookiePair(cookiePair: string, looseMode: boolean) {
+function parseCookiePair(
+  cookiePair: string,
+  looseMode: boolean,
+): Cookie | undefined {
   cookiePair = trimTerminator(cookiePair)
   validators.validate(validators.isString(cookiePair), cookiePair)
 
@@ -284,7 +287,7 @@ function parse(
   return c
 }
 
-function fromJSON(str: unknown) {
+function fromJSON(str: unknown): Cookie | null {
   if (!str || validators.isEmptyString(str)) {
     return null
   }
@@ -452,7 +455,7 @@ export class Cookie {
     this.creationIndex = Cookie.cookiesCreated
   }
 
-  [Symbol.for('nodejs.util.inspect.custom')]() {
+  [Symbol.for('nodejs.util.inspect.custom')](): string {
     const now = Date.now()
     const hostOnly = this.hostOnly != null ? this.hostOnly.toString() : '?'
     const createAge =
@@ -536,11 +539,11 @@ export class Cookie {
     return obj
   }
 
-  clone() {
+  clone(): Cookie | null {
     return fromJSON(this.toJSON())
   }
 
-  validate() {
+  validate(): boolean {
     if (this.value == null || !COOKIE_OCTETS.test(this.value)) {
       return false
     }
@@ -576,7 +579,7 @@ export class Cookie {
     return true
   }
 
-  setExpires(exp: string | Date) {
+  setExpires(exp: string | Date): void {
     if (exp instanceof Date) {
       this.expires = exp
     } else {
@@ -584,7 +587,7 @@ export class Cookie {
     }
   }
 
-  setMaxAge(age: number) {
+  setMaxAge(age: number): void {
     if (age === Infinity) {
       this.maxAge = 'Infinity'
     } else if (age === -Infinity) {
@@ -594,7 +597,7 @@ export class Cookie {
     }
   }
 
-  cookieString() {
+  cookieString(): string {
     const val = this.value ?? ''
     if (this.key) {
       return `${this.key}=${val}`
@@ -603,7 +606,7 @@ export class Cookie {
   }
 
   // gives Set-Cookie header format
-  toString() {
+  toString(): string {
     let str = this.cookieString()
 
     if (this.expires != 'Infinity') {
@@ -701,14 +704,14 @@ export class Cookie {
   }
 
   // Mostly S5.1.2 and S5.2.3:
-  canonicalizedDomain() {
+  canonicalizedDomain(): string | null {
     if (this.domain == null) {
       return null
     }
     return canonicalDomain(this.domain)
   }
 
-  cdomain() {
+  cdomain(): string | null {
     return this.canonicalizedDomain()
   }
 
diff --git a/lib/cookie/cookieCompare.ts b/lib/cookie/cookieCompare.ts
index dd793f4e..f5a1a16a 100644
--- a/lib/cookie/cookieCompare.ts
+++ b/lib/cookie/cookieCompare.ts
@@ -17,7 +17,7 @@ import type { Cookie } from './cookie'
 const MAX_TIME = 2147483647000
 
 /** Compares two cookies for sorting. */
-export function cookieCompare(a: Cookie, b: Cookie) {
+export function cookieCompare(a: Cookie, b: Cookie): number {
   validators.validate(validators.isObject(a), safeToString(a))
   validators.validate(validators.isObject(b), safeToString(b))
   let cmp: number
diff --git a/lib/cookie/cookieJar.ts b/lib/cookie/cookieJar.ts
index 11e6af71..f638d28f 100644
--- a/lib/cookie/cookieJar.ts
+++ b/lib/cookie/cookieJar.ts
@@ -70,7 +70,7 @@ type CreateCookieJarOptions = {
 const SAME_SITE_CONTEXT_VAL_ERR =
   'Invalid sameSiteContext option for getCookies(); expected one of "strict", "lax", or "none"'
 
-function getCookieContext(url: unknown) {
+function getCookieContext(url: unknown): URL | urlParse<string> {
   if (url instanceof URL) {
     return url
   } else if (typeof url === 'string') {
@@ -84,7 +84,8 @@ function getCookieContext(url: unknown) {
   }
 }
 
-function checkSameSiteContext(value: string) {
+type SameSiteLevel = keyof (typeof Cookie)['sameSiteLevel']
+function checkSameSiteContext(value: string): SameSiteLevel | null {
   validators.validate(validators.isNonEmptyString(value), value)
   const context = String(value).toLowerCase()
   if (context === 'none' || context === 'lax' || context === 'strict') {
@@ -101,7 +102,7 @@ function checkSameSiteContext(value: string) {
  * @param cookie
  * @returns boolean
  */
-function isSecurePrefixConditionMet(cookie: Cookie) {
+function isSecurePrefixConditionMet(cookie: Cookie): boolean {
   validators.validate(validators.isObject(cookie), safeToString(cookie))
   const startsWithSecurePrefix =
     typeof cookie.key === 'string' && cookie.key.startsWith('__Secure-')
@@ -119,20 +120,26 @@ function isSecurePrefixConditionMet(cookie: Cookie) {
  * @param cookie
  * @returns boolean
  */
-function isHostPrefixConditionMet(cookie: Cookie) {
+function isHostPrefixConditionMet(cookie: Cookie): boolean {
   validators.validate(validators.isObject(cookie))
   const startsWithHostPrefix =
     typeof cookie.key === 'string' && cookie.key.startsWith('__Host-')
   return (
     !startsWithHostPrefix ||
-    (cookie.secure &&
-      cookie.hostOnly &&
-      cookie.path != null &&
-      cookie.path === '/')
+    Boolean(
+      cookie.secure &&
+        cookie.hostOnly &&
+        cookie.path != null &&
+        cookie.path === '/',
+    )
   )
 }
 
-function getNormalizedPrefixSecurity(prefixSecurity: string) {
+type PrefixSecurityValue =
+  (typeof PrefixSecurityEnum)[keyof typeof PrefixSecurityEnum]
+function getNormalizedPrefixSecurity(
+  prefixSecurity: string,
+): PrefixSecurityValue {
   if (prefixSecurity != null) {
     const normalizedPrefixSecurity = prefixSecurity.toLowerCase()
     /* The three supported options */
@@ -563,7 +570,7 @@ export class CookieJar {
     const allPaths = options?.allPaths ?? false
     const store = this.store
 
-    function matchingCookie(c: Cookie) {
+    function matchingCookie(c: Cookie): boolean {
       // "Either:
       //   The cookie's host-only-flag is true and the canonicalized
       //   request-host is identical to the cookie's domain.
@@ -843,12 +850,12 @@ export class CookieJar {
     return this.callSync((callback) => this.serialize(callback))
   }
 
-  toJSON() {
+  toJSON(): SerializedCookieJar | undefined {
     return this.serializeSync()
   }
 
   // use the class method CookieJar.deserialize instead of calling this directly
-  _importCookies(serialized: unknown, callback: Callback<CookieJar>) {
+  _importCookies(serialized: unknown, callback: Callback<CookieJar>): void {
     let cookies: unknown[] | undefined = undefined
 
     if (
diff --git a/lib/cookie/formatDate.ts b/lib/cookie/formatDate.ts
index c039c1da..faa93f7e 100644
--- a/lib/cookie/formatDate.ts
+++ b/lib/cookie/formatDate.ts
@@ -2,7 +2,7 @@ import * as validators from '../validators'
 import { safeToString } from '../utils'
 
 /** Converts a Date to a UTC string representation. */
-export function formatDate(date: Date) {
+export function formatDate(date: Date): string {
   validators.validate(validators.isDate(date), safeToString(date))
   return date.toUTCString()
 }
diff --git a/lib/cookie/parseDate.ts b/lib/cookie/parseDate.ts
index e3963d1b..3ffc1ace 100644
--- a/lib/cookie/parseDate.ts
+++ b/lib/cookie/parseDate.ts
@@ -35,7 +35,7 @@ function parseDigits(
   minDigits: number,
   maxDigits: number,
   trailingOK: boolean,
-) {
+): number | null {
   let count = 0
   while (count < token.length) {
     const c = token.charCodeAt(count)
@@ -58,7 +58,7 @@ function parseDigits(
   return parseInt(token.slice(0, count), 10)
 }
 
-function parseTime(token: string) {
+function parseTime(token: string): number[] | null {
   const parts = token.split(':')
   const result = [0, 0, 0]
 
@@ -91,7 +91,7 @@ function parseTime(token: string) {
   return result
 }
 
-function parseMonth(token: string) {
+function parseMonth(token: string): number | null {
   token = String(token).slice(0, 3).toLowerCase()
   switch (token) {
     case 'jan':
diff --git a/lib/memstore.ts b/lib/memstore.ts
index 3f04372a..e66a683f 100644
--- a/lib/memstore.ts
+++ b/lib/memstore.ts
@@ -119,7 +119,7 @@ export class MemoryCookieStore extends Store {
     ) => void
     if (!path) {
       // null means "all paths"
-      pathMatcher = function matchAll(domainIndex) {
+      pathMatcher = function matchAll(domainIndex): void {
         for (const curPath in domainIndex) {
           const pathIndex = domainIndex[curPath]
           for (const key in pathIndex) {
@@ -131,7 +131,7 @@ export class MemoryCookieStore extends Store {
         }
       }
     } else {
-      pathMatcher = function matchRFC(domainIndex) {
+      pathMatcher = function matchRFC(domainIndex): void {
         //NOTE: we should use path-match algorithm from S5.1.4 here
         //(see : https://github.com/ChromiumWebApps/chromium/blob/b3d3b4da8bb94c1b2e061600df106d590fda3620/net/cookies/canonical_cookie.cc#L299)
         for (const cookiePath in domainIndex) {
diff --git a/lib/utils.ts b/lib/utils.ts
index 8f4ab317..c3068b7d 100644
--- a/lib/utils.ts
+++ b/lib/utils.ts
@@ -13,7 +13,7 @@ export interface ErrorCallback {
 export type Nullable<T> = T | null | undefined
 
 /** Wrapped `Object.prototype.toString`, so that you don't need to remember to use `.call()`. */
-export const objectToString = (obj: unknown) =>
+export const objectToString = (obj: unknown): string =>
   Object.prototype.toString.call(obj)
 
 /**
@@ -52,7 +52,7 @@ const safeToStringImpl = (
 }
 
 /** Safely converts any value to string, using the value's own `toString` when available. */
-export const safeToString = (val: unknown) => safeToStringImpl(val)
+export const safeToString = (val: unknown): string => safeToStringImpl(val)
 
 /** Utility object for promise/callback interop. */
 export interface PromiseCallback<T> {
@@ -74,7 +74,7 @@ export function createPromiseCallback<T>(cb?: Callback<T>): PromiseCallback<T> {
   })
 
   if (typeof cb === 'function') {
-    callback = (err, result) => {
+    callback = (err, result): void => {
       try {
         if (err) cb(err)
         // If `err` is null, we know `result` must be `T`
@@ -86,7 +86,7 @@ export function createPromiseCallback<T>(cb?: Callback<T>): PromiseCallback<T> {
       }
     }
   } else {
-    callback = (err, result) => {
+    callback = (err, result): void => {
       try {
         // If `err` is null, we know `result` must be `T`
         // The assertion isn't *strictly* correct, as `T` could be nullish, but, ehh, good enough...
@@ -101,11 +101,11 @@ export function createPromiseCallback<T>(cb?: Callback<T>): PromiseCallback<T> {
   return {
     promise,
     callback,
-    resolve: (value: T) => {
+    resolve: (value: T): Promise<T> => {
       callback(null, value)
       return promise
     },
-    reject: (error: Error) => {
+    reject: (error: Error): Promise<T> => {
       callback(error)
       return promise
     },