diff --git a/explorer/package.json b/explorer/package.json
index c3ce6ff..be3eb8c 100644
--- a/explorer/package.json
+++ b/explorer/package.json
@@ -85,11 +85,17 @@
     ]
   },
   "babel": {
+    "plugins": [
+      "flow-react-proptypes"
+    ],
     "presets": [
       "react-app"
     ]
   },
   "eslintConfig": {
     "extends": "react-app"
+  },
+  "devDependencies": {
+    "babel-plugin-flow-react-proptypes": "^17.1.2"
   }
 }
diff --git a/explorer/src/App.js b/explorer/src/App.js
index 51d1081..19c182f 100644
--- a/explorer/src/App.js
+++ b/explorer/src/App.js
@@ -1,10 +1,12 @@
+// @flow
 import React, { Component } from 'react';
 import data from './data.json';
 import './App.css';
 import { FileExplorer } from './FileExplorer.js';
 import { UserExplorer } from './UserExplorer.js';
 
-class App extends Component {
+type AppState = {selectedPath: string, selectedUser: ?string};
+class App extends Component<{}, AppState> {
   constructor() {
     super();
     this.state = {
diff --git a/explorer/src/FileExplorer.js b/explorer/src/FileExplorer.js
index 41f11cc..34a715b 100644
--- a/explorer/src/FileExplorer.js
+++ b/explorer/src/FileExplorer.js
@@ -1,15 +1,13 @@
+// @flow
 import React, { Component } from 'react';
-import PropTypes from 'prop-types';
 import {buildTree} from './commitUtils';
-import {propTypes as commitUtilsPropTypes} from './commitUtils';
-
-export class FileExplorer extends Component {
-  static propTypes = {
-    selectedPath: PropTypes.string,
-    onSelectPath: PropTypes.func.isRequired,
-    data: commitUtilsPropTypes.commitData.isRequired,
-  }
+import type {CommitData, FileTree} from './commitUtils';
 
+export class FileExplorer extends Component<{
+    selectedPath: string,
+    onSelectPath: (newPath: string) => void,
+    data: CommitData,
+}> {
   render() {
     // within the FileExplorer, paths start with "./", outside they don't
     // which is hacky and should be cleaned up
@@ -38,19 +36,16 @@ export class FileExplorer extends Component {
   }
 }
 
-class FileEntry extends Component {
-  static propTypes = {
-    name: PropTypes.string.isRequired,
-    path: PropTypes.string.isRequired,
-    alwaysExpand: PropTypes.bool.isRequired,
-
-    // The type for the tree is recursive, and is annoying to specify as
-    // a proptype. The Flow type definition is in commitUtils.js.
-    tree: PropTypes.object.isRequired,
-
-    selectedPath: PropTypes.string.isRequired,
-    onSelectPath: PropTypes.func.isRequired,
-  }
+class FileEntry extends Component<{
+  name: string,
+  path: string,
+  alwaysExpand: bool,
+  tree: FileTree,
+  selectedPath: string,
+  onSelectPath: (newPath: string) => void,
+}, {
+  expanded: bool,
+}> {
 
   constructor() {
     super();
@@ -89,4 +84,5 @@ class FileEntry extends Component {
       {(this.state.expanded || this.props.alwaysExpand) && subEntries}
     </div>
   }
+
 }
diff --git a/explorer/src/UserExplorer.js b/explorer/src/UserExplorer.js
index 8da252d..bb574cd 100644
--- a/explorer/src/UserExplorer.js
+++ b/explorer/src/UserExplorer.js
@@ -1,18 +1,14 @@
+// @flow
 import React, { Component } from 'react';
-import PropTypes from 'prop-types';
-import {
-  commitWeight,
-  propTypes as commitUtilsPropTypes,
-  userWeightForPath,
-} from './commitUtils';
+import {commitWeight, userWeightForPath} from './commitUtils';
+import type {CommitData, FileTree} from './commitUtils';
 
-export class UserExplorer extends Component {
-  static propTypes = {
-    selectedPath: PropTypes.string.isRequired,
-    selectedUser: PropTypes.string,
-    onSelectUser: PropTypes.func.isRequired,
-    data: commitUtilsPropTypes.commitData.isRequired,
-  }
+export class UserExplorer extends Component<{
+  selectedPath: string,
+  selectedUser: ?string,
+  onSelectUser: (newUser: string) => void,
+  data: CommitData,
+}> {
 
   render() {
     const weights = userWeightForPath(this.props.selectedPath, this.props.data, commitWeight);
@@ -29,16 +25,16 @@ export class UserExplorer extends Component {
       {entries}
     </div>
   }
+
 }
 
 /**
  * Record the cred earned by the user in a given scope.
  */
-class UserEntry extends Component {
-  static propTypes = {
-    userId: PropTypes.string.isRequired,
-    weight: PropTypes.number.isRequired,
-  }
+class UserEntry extends Component<{
+    userId: string,
+    weight: number,
+}> {
 
   render() {
     return <div className="user-entry">
@@ -46,4 +42,5 @@ class UserEntry extends Component {
       <span> {this.props.weight.toFixed(1)} </span>
     </div>
   }
+
 }
diff --git a/explorer/src/commitUtils.js b/explorer/src/commitUtils.js
index 4cca4a0..870ec0d 100644
--- a/explorer/src/commitUtils.js
+++ b/explorer/src/commitUtils.js
@@ -2,30 +2,12 @@
 
 import PropTypes from 'prop-types';
 
-type CommitData = {
+export type CommitData = {
   // TODO improve variable names
   fileToCommits: {[filename: string]: string[]};
   commits: {[commithash: string]: Commit};
-  authors: string[];
 }
 
-export const propTypes = {
-  commitData: PropTypes.shape({
-    fileToCommits: PropTypes.objectOf(
-      PropTypes.arrayOf(PropTypes.string.isRequired).isRequired,
-    ).isRequired,
-    commits: PropTypes.objectOf(PropTypes.shape({
-      author: PropTypes.string.isRequired,
-      stats: PropTypes.objectOf(PropTypes.shape({
-        lines: PropTypes.number.isRequired,
-        insertions: PropTypes.number.isRequired,
-        deletions: PropTypes.number.isRequired,
-      }).isRequired).isRequired,
-    }).isRequired).isRequired,
-  }),
-};
-
-
 type Commit = {
   author: string;
   stats: {[filename: string]: FileStats};
@@ -86,7 +68,7 @@ export function userWeightForPath(path: string, data: CommitData, weightFn: Weig
   return userWeightMap;
 }
 
-type FileTree = {[string]: FileTree};
+export type FileTree = {[string]: FileTree};
 
 export function buildTree(fileNames: string[]): FileTree {
   const sortedFileNames = fileNames.slice().sort();
diff --git a/explorer/yarn.lock b/explorer/yarn.lock
index 019bb7e..bc75ae0 100644
--- a/explorer/yarn.lock
+++ b/explorer/yarn.lock
@@ -323,7 +323,7 @@ babel-code-frame@6.26.0, babel-code-frame@^6.11.0, babel-code-frame@^6.22.0, bab
     esutils "^2.0.2"
     js-tokens "^3.0.2"
 
-babel-core@6.26.0, babel-core@^6.0.0, babel-core@^6.26.0:
+babel-core@6.26.0, babel-core@^6.0.0, babel-core@^6.25.0, babel-core@^6.26.0:
   version "6.26.0"
   resolved "https://registry.yarnpkg.com/babel-core/-/babel-core-6.26.0.tgz#af32f78b31a6fcef119c87b0fd8d9753f03a0bb8"
   dependencies:
@@ -514,6 +514,15 @@ babel-plugin-dynamic-import-node@1.1.0:
     babel-template "^6.26.0"
     babel-types "^6.26.0"
 
+babel-plugin-flow-react-proptypes@^17.1.2:
+  version "17.1.2"
+  resolved "https://registry.yarnpkg.com/babel-plugin-flow-react-proptypes/-/babel-plugin-flow-react-proptypes-17.1.2.tgz#89f75928a47ea869dab312605f42542dd7b6755c"
+  dependencies:
+    babel-core "^6.25.0"
+    babel-template "^6.25.0"
+    babel-traverse "^6.25.0"
+    babel-types "^6.25.0"
+
 babel-plugin-istanbul@^4.0.0:
   version "4.1.5"
   resolved "https://registry.yarnpkg.com/babel-plugin-istanbul/-/babel-plugin-istanbul-4.1.5.tgz#6760cdd977f411d3e175bb064f2bc327d99b2b6e"
@@ -913,7 +922,7 @@ babel-runtime@6.26.0, babel-runtime@^6.18.0, babel-runtime@^6.22.0, babel-runtim
     core-js "^2.4.0"
     regenerator-runtime "^0.11.0"
 
-babel-template@^6.16.0, babel-template@^6.24.1, babel-template@^6.26.0:
+babel-template@^6.16.0, babel-template@^6.24.1, babel-template@^6.25.0, babel-template@^6.26.0:
   version "6.26.0"
   resolved "https://registry.yarnpkg.com/babel-template/-/babel-template-6.26.0.tgz#de03e2d16396b069f46dd9fff8521fb1a0e35e02"
   dependencies:
@@ -923,7 +932,7 @@ babel-template@^6.16.0, babel-template@^6.24.1, babel-template@^6.26.0:
     babylon "^6.18.0"
     lodash "^4.17.4"
 
-babel-traverse@^6.18.0, babel-traverse@^6.23.1, babel-traverse@^6.24.1, babel-traverse@^6.26.0:
+babel-traverse@^6.18.0, babel-traverse@^6.23.1, babel-traverse@^6.24.1, babel-traverse@^6.25.0, babel-traverse@^6.26.0:
   version "6.26.0"
   resolved "https://registry.yarnpkg.com/babel-traverse/-/babel-traverse-6.26.0.tgz#46a9cbd7edcc62c8e5c064e2d2d8d0f4035766ee"
   dependencies:
@@ -937,7 +946,7 @@ babel-traverse@^6.18.0, babel-traverse@^6.23.1, babel-traverse@^6.24.1, babel-tr
     invariant "^2.2.2"
     lodash "^4.17.4"
 
-babel-types@^6.18.0, babel-types@^6.19.0, babel-types@^6.23.0, babel-types@^6.24.1, babel-types@^6.26.0:
+babel-types@^6.18.0, babel-types@^6.19.0, babel-types@^6.23.0, babel-types@^6.24.1, babel-types@^6.25.0, babel-types@^6.26.0:
   version "6.26.0"
   resolved "https://registry.yarnpkg.com/babel-types/-/babel-types-6.26.0.tgz#a3b073f94ab49eb6fa55cd65227a334380632497"
   dependencies: