diff --git a/experimental/babel-preset-env/.eslintignore b/experimental/babel-preset-env/.eslintignore
index 87bef84070..8af3c04471 100644
--- a/experimental/babel-preset-env/.eslintignore
+++ b/experimental/babel-preset-env/.eslintignore
@@ -1,3 +1,4 @@
 /lib
 fixtures
 /data
+/flow-typed
diff --git a/experimental/babel-preset-env/.flowconfig b/experimental/babel-preset-env/.flowconfig
new file mode 100644
index 0000000000..4a62bf66c4
--- /dev/null
+++ b/experimental/babel-preset-env/.flowconfig
@@ -0,0 +1,10 @@
+[ignore]
+.*/lib/.*
+.*/test/.*
+.*/build/.*
+.*/node_modules/jest-validate/.*
+
+[options]
+strip_root=true
+suppress_comment= \\(.\\|\n\\)*\\$FlowFixMe
+suppress_comment= \\(.\\|\n\\)*\\$FlowIssue
diff --git a/experimental/babel-preset-env/.npmignore b/experimental/babel-preset-env/.npmignore
index bf67f6570e..860048ba15 100644
--- a/experimental/babel-preset-env/.npmignore
+++ b/experimental/babel-preset-env/.npmignore
@@ -9,3 +9,4 @@ yarn.lock
 .nyc_output
 .vscode
 babel-preset-env-*.tgz
+flow-typed
diff --git a/experimental/babel-preset-env/flow-typed/npm/semver_v5.1.x.js b/experimental/babel-preset-env/flow-typed/npm/semver_v5.1.x.js
new file mode 100644
index 0000000000..6331b8488c
--- /dev/null
+++ b/experimental/babel-preset-env/flow-typed/npm/semver_v5.1.x.js
@@ -0,0 +1,81 @@
+// flow-typed signature: c5f918cd3de18b19a20558e6f3bbcc84
+// flow-typed version: cdd17a64e0/semver_v5.1.x/flow_>=v0.27.0
+
+// List of members taken from here: https://www.npmjs.com/package/semver/#functions
+// TODO support the `loose` parameter
+// TODO support SemVer instances as input parameters
+declare module 'semver' {
+  declare type Release =
+    'major' |
+    'premajor' |
+    'minor' |
+    'preminor' |
+    'patch' |
+    'prepatch' |
+    'prerelease';
+
+  // The supported comparators are taken from the source here:
+  // https://github.com/npm/node-semver/blob/8bd070b550db2646362c9883c8d008d32f66a234/semver.js#L623
+  declare type Comparator =
+    '===' |
+    '!==' |
+    '==' |
+    '=' |
+    '' |  // Not sure why you would want this, but whatever.
+    '!=' |
+    '>' |
+    '>=' |
+    '<' |
+    '<=';
+
+  declare class SemVer {
+    loose: ?boolean,
+    raw: string,
+    major: number,
+    minor: number,
+    patch: number,
+    prerelease: Array<string | number>,
+    build: Array<string>,
+    version: string,
+  }
+
+  // Functions
+  declare function valid(v: string): string | null;
+  declare function inc(v: string, release: Release): string | null;
+  declare function major(v: string): number;
+  declare function minor(v: string): number;
+  declare function patch(v: string): number;
+
+  // Comparison
+  declare function gt(v1: string, v2: string): boolean;
+  declare function gte(v1: string, v2: string): boolean;
+  declare function lt(v1: string, v2: string): boolean;
+  declare function lte(v1: string, v2: string): boolean;
+  declare function eq(v1: string, v2: string): boolean;
+  declare function neq(v1: string, v2: string): boolean;
+  declare function cmp(v1: string, comparator: Comparator, v2: string): boolean;
+  declare function compare(v1: string, v2: string): -1 | 0 | 1;
+  declare function rcompare(v1: string, v2: string): -1 | 0 | 1;
+  declare function diff(v1: string, v2: string): ?Release;
+
+  // Ranges
+  declare function validRange(r: string): string | null;
+  declare function satisfies(version: string, range: string): boolean;
+  declare function maxSatisfying(versions: Array<string>, range: string): string | null;
+  declare function gtr(version: string, range: string): boolean;
+  declare function ltr(version: string, range: string): boolean;
+  declare function outside(version: string, range: string, hilo: '>' | '<'): boolean;
+
+  // Not explicitly documented
+  declare function parse(version: string): ?SemVer;
+
+  declare class Range {
+    set: Array<Array<{semver: SemVer}>>;
+
+    constructor(range: string, loose?: boolean): Range;
+
+    format(): string;
+    test(version: string): boolean;
+    toString(): string;
+  }
+}
diff --git a/experimental/babel-preset-env/package.json b/experimental/babel-preset-env/package.json
index 4179182b8b..6e6401065f 100644
--- a/experimental/babel-preset-env/package.json
+++ b/experimental/babel-preset-env/package.json
@@ -15,11 +15,12 @@
     "coverage-ci": "nyc report --reporter=json && codecov -f coverage/coverage-final.json",
     "dev": "babel -w src -d lib",
     "fix": "eslint . --fix",
+    "flow": "flow",
     "format": "prettier --trailing-comma all --write \"src/*.js\" \"test/*.js\" && prettier --trailing-comma es5 --write \"scripts/*.js\"",
     "lint": "eslint .",
     "precommit": "lint-staged",
     "prepublish": "npm run build",
-    "test": "npm run build && npm run test-only",
+    "test": "npm run build && npm run flow && npm run test-only",
     "test-ci": "nyc npm run test",
     "test-only": "mocha ./test --compilers js:babel-register -t 10000"
   },
@@ -71,6 +72,7 @@
     "eslint": "^3.17.1",
     "eslint-config-babel": "^6.0.0",
     "eslint-plugin-flowtype": "^2.29.1",
+    "flow-bin": "^0.43.0",
     "fs-extra": "^2.0.0",
     "husky": "^0.13.2",
     "lint-staged": "^3.3.1",
diff --git a/experimental/babel-preset-env/src/targets-parser.js b/experimental/babel-preset-env/src/targets-parser.js
index 1a8cbf7551..b7025a3fa4 100644
--- a/experimental/babel-preset-env/src/targets-parser.js
+++ b/experimental/babel-preset-env/src/targets-parser.js
@@ -1,7 +1,13 @@
+// @flow
+
 import browserslist from "browserslist";
 import semver from "semver";
 import { semverify } from "./utils";
 
+export type Targets = {
+  [target: string]: string,
+};
+
 const browserNameMap = {
   chrome: "chrome",
   edge: "edge",
@@ -11,10 +17,10 @@ const browserNameMap = {
   safari: "safari",
 };
 
-const isBrowsersQueryValid = browsers =>
+const isBrowsersQueryValid = (browsers: string | Array<string>): boolean =>
   typeof browsers === "string" || Array.isArray(browsers);
 
-const semverMin = (first, second) => {
+const semverMin = (first: ?string, second: string): string => {
   return first && semver.lt(first, second) ? first : second;
 };
 
@@ -45,7 +51,7 @@ const getLowestVersions = browsers => {
   );
 };
 
-const outputDecimalWarning = decimalTargets => {
+const outputDecimalWarning = (decimalTargets: Array<Object>): void => {
   if (!decimalTargets || !decimalTargets.length) {
     return;
   }
@@ -78,7 +84,7 @@ const targetParserMap = {
   uglify: (target, value) => [target, value === true],
 };
 
-const getTargets = (targets = {}) => {
+const getTargets = (targets: Object = {}): Targets => {
   let targetOpts = {};
 
   // Parse browsers target via browserslist
diff --git a/experimental/babel-preset-env/src/utils.js b/experimental/babel-preset-env/src/utils.js
index 54d37b8287..db59f4301c 100644
--- a/experimental/babel-preset-env/src/utils.js
+++ b/experimental/babel-preset-env/src/utils.js
@@ -1,9 +1,11 @@
 // @flow
+
 import semver from "semver";
+import type { Targets } from "./targets-parser";
 
 // Convert version to a semver value.
 // 2.5 -> 2.5.0; 1 -> 1.0.0;
-export const semverify = version => {
+export const semverify = (version: string | number) => {
   if (typeof version === "string" && semver.valid(version)) {
     return version;
   }
@@ -11,13 +13,13 @@ export const semverify = version => {
   const split = version.toString().split(".");
 
   while (split.length < 3) {
-    split.push(0);
+    split.push("0");
   }
 
   return split.join(".");
 };
 
-export const prettifyVersion = version => {
+export const prettifyVersion = (version: string | number) => {
   if (typeof version !== "string") {
     return version;
   }
@@ -37,7 +39,7 @@ export const prettifyVersion = version => {
   return parts.join(".");
 };
 
-export const prettifyTargets = (targets = {}) => {
+export const prettifyTargets = (targets: Targets) => {
   return Object.keys(targets).reduce(
     (results, target) => {
       let value = targets[target];
diff --git a/experimental/babel-preset-env/yarn.lock b/experimental/babel-preset-env/yarn.lock
index a178bb3434..54a5591b1d 100644
--- a/experimental/babel-preset-env/yarn.lock
+++ b/experimental/babel-preset-env/yarn.lock
@@ -2637,6 +2637,10 @@ flat-cache@^1.2.1:
     graceful-fs "^4.1.2"
     write "^0.2.1"
 
+flow-bin@^0.43.0:
+  version "0.43.1"
+  resolved "https://registry.yarnpkg.com/flow-bin/-/flow-bin-0.43.1.tgz#0733958b448fb8ad4b1576add7e87c31794c81bc"
+
 flow-parser@0.40.0:
   version "0.40.0"
   resolved "https://registry.yarnpkg.com/flow-parser/-/flow-parser-0.40.0.tgz#b3444742189093323c4319c4fe9d35391f46bcbc"