diff --git a/src/__tests__/flatToMdast.spec.ts b/src/__tests__/flatToMdast.spec.ts
new file mode 100644
index 00000000..b7944fc5
--- /dev/null
+++ b/src/__tests__/flatToMdast.spec.ts
@@ -0,0 +1,261 @@
+import {flatToMdast} from '../flatToMdast';
+
+describe('structure', () => {
+  it('exists', () => {
+    expect(typeof flatToMdast).toBe('function');
+  });
+
+  it('single root', () => {
+    const flat = {
+      nodes: [
+        {
+          type: 'root',
+        }
+      ]
+    };
+    const mdast = flatToMdast(flat as any);
+
+    expect(mdast).toEqual({
+      type: 'root',
+      children: [],
+    });
+  });
+
+  it('doc 1', () => {
+    const flat = {
+      nodes: [
+        {
+          type: 'root',
+          children: [1],
+        },
+        {
+          type: 'paragraph',
+          children: [2],
+        },
+        {
+          type: 'text',
+          value: 'foo',
+        },
+      ],
+      contents: [],
+      definitions: {},
+      footnotes: {},
+    };
+    const mdast = flatToMdast(flat as any);
+
+    expect(mdast).toEqual({
+      type: 'root',
+      children: [
+        {
+          type: 'paragraph',
+          children: [
+            {
+              type: 'text',
+              value: 'foo',
+            },
+          ],
+        },
+      ],
+    });
+  });
+
+  it('doc 2', () => {
+    const flat = {
+      nodes: [
+        {
+          type: 'root',
+          children: [1, 3],
+        },
+        {
+          type: 'heading',
+          children: [2],
+          depth: 1,
+        },
+        {
+          type: 'text',
+          value: 'Title',
+        },
+        {
+          type: 'heading',
+          children: [4],
+          depth: 2,
+        },
+        {
+          type: 'text',
+          value: 'Subtitle',
+        },
+      ],
+      contents: [1, 3],
+      definitions: {},
+      footnotes: {},
+    };
+    const mdast = flatToMdast(flat as any);
+
+    expect(mdast).toEqual({
+      type: 'root',
+      children: [
+        {
+          type: 'heading',
+          depth: 1,
+          children: [
+            {
+              type: 'text',
+              value: 'Title',
+            },
+          ],
+        },
+        {
+          type: 'heading',
+          depth: 2,
+          children: [
+            {
+              type: 'text',
+              value: 'Subtitle',
+            },
+          ],
+        },
+      ],
+    });
+  });
+
+  it('doc 3', () => {
+    const flat = {
+      nodes: [
+        {
+          type: 'root',
+          children: [1],
+        },
+        {
+          type: 'paragraph',
+          children: [2],
+        },
+        {
+          type: 'linkReference',
+          children: [3],
+          identifier: 'click',
+          referenceType: 'full',
+        },
+        {
+          type: 'text',
+          value: 'Click me',
+        },
+        {
+          type: 'definition',
+          identifier: 'click',
+          title: null,
+          url: 'https://github.com/',
+        },
+      ],
+      contents: [],
+      definitions: {
+        click: 4,
+      },
+      footnotes: {},
+    };
+    const mdast = flatToMdast(flat as any);
+
+    expect(mdast).toEqual({
+      type: 'root',
+      children: [
+        {
+          type: 'paragraph',
+          children: [
+            {
+              type: 'linkReference',
+              children: [
+                {
+                  type: 'text',
+                  value: 'Click me',
+                },
+              ],
+              identifier: 'click',
+              referenceType: 'full',
+            },
+          ],
+        },
+        {
+          type: 'definition',
+          identifier: 'click',
+          title: null,
+          url: 'https://github.com/',
+        },
+      ],
+    });
+  });
+
+  it('doc 4', () => {
+    const flat = {
+      nodes: [
+        {
+          type: 'root',
+          children: [1],
+        },
+        {
+          type: 'paragraph',
+          children: [2, 3],
+        },
+        {
+          type: 'text',
+          value: 'Hello',
+        },
+        {
+          type: 'footnoteReference',
+          value: 'gg',
+        },
+        {
+          type: 'footnoteDefinition',
+          children: [5],
+          identifier: 'gg',
+        },
+        {
+          type: 'paragraph',
+          children: [6],
+        },
+        {
+          type: 'text',
+          value: 'world!',
+        },
+      ],
+      contents: [],
+      definitions: {},
+      footnotes: {
+        gg: 4,
+      },
+    };
+    const mdast = flatToMdast(flat as any);
+
+    expect(mdast).toEqual({
+      type: 'root',
+      children: [
+        {
+          type: 'paragraph',
+          children: [
+            {
+              type: 'text',
+              value: 'Hello',
+            },
+            {
+              type: 'footnoteReference',
+              value: 'gg',
+            },
+          ],
+        },
+        {
+          type: 'footnoteDefinition',
+          identifier: 'gg',
+          children: [
+            {
+              type: 'paragraph',
+              children: [
+                {
+                  type: 'text',
+                  value: 'world!'
+                },
+              ],
+            },
+          ],
+        },
+      ],
+    });
+  });
+});
diff --git a/src/flatToMdast.ts b/src/flatToMdast.ts
new file mode 100644
index 00000000..ea54f798
--- /dev/null
+++ b/src/flatToMdast.ts
@@ -0,0 +1,26 @@
+import {FlatToMdast, Flat} from './types';
+import {IRoot, TAnyToken, TBlockToken} from 'md-mdast/lib/types';
+
+export const flatToMdast: FlatToMdast = (flat: Flat) => {
+  const traverse: (index: number) => IRoot | TAnyToken = (index) => {
+    const node: any = {...flat.nodes[index]};
+    if (node.children) node.children = node.children.map(traverse);
+    return node;
+  };
+
+  const mdast = traverse(0) as IRoot;
+  if (!mdast.children) mdast.children = [];
+
+  if (flat.definitions instanceof Object) {
+    Object.values(flat.definitions).forEach(index => {
+      mdast.children.push(traverse(index) as TBlockToken);
+    });
+  }
+  if (flat.footnotes instanceof Object) {
+    Object.values(flat.footnotes).forEach(index => {
+      mdast.children.push(traverse(index) as TBlockToken);
+    });
+  }
+
+  return mdast;
+};
diff --git a/src/types.ts b/src/types.ts
index 5d999850..7e7f9ef8 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -18,3 +18,4 @@ export interface Flat {
 }
 
 export type MdastToFlat = (mdast: IRoot | TAnyToken) => Flat;
+export type FlatToMdast = (flat: Flat) => IRoot | TAnyToken;