diff --git a/lib/third-party-libs.js.flow b/lib/third-party-libs.js.flow
index c21a2797acfe..884b1e68bbfb 100644
--- a/lib/third-party-libs.js.flow
+++ b/lib/third-party-libs.js.flow
@@ -165,7 +165,7 @@ declare module "source-map" {
 }
 
 declare module "convert-source-map" {
-  import type { SourceMap } from "source-map";
+  import type { SourceMap, SourceMapGenerator } from "source-map";
 
   declare class Converter {
     toJSON(): string;
@@ -177,7 +177,7 @@ declare module "convert-source-map" {
   declare module.exports: {
     SourceMap: SourceMap,
     Converter: Converter,
-    fromObject(obj: SourceMap): Converter,
+    fromObject(obj: SourceMap | SourceMapGenerator): Converter,
     fromJSON(str: string): Converter,
     fromBase64(str: string): Converter,
     fromComment(str: string): Converter,
diff --git a/packages/babel-cli/src/babel-external-helpers.js b/packages/babel-cli/src/babel-external-helpers.js
index c2560dfa6f37..fef536187c41 100755
--- a/packages/babel-cli/src/babel-external-helpers.js
+++ b/packages/babel-cli/src/babel-external-helpers.js
@@ -1,7 +1,12 @@
+// @flow
+
 import commander from "commander";
 import { buildExternalHelpers } from "@babel/core";
 
-function collect(value, previousValue): Array<string> {
+function collect(
+  value: string | any,
+  previousValue: Array<string>,
+): Array<string> {
   // If the user passed the option with no value, like "babel-external-helpers --whitelist", do nothing.
   if (typeof value !== "string") return previousValue;
 
diff --git a/packages/babel-cli/src/babel/dir.js b/packages/babel-cli/src/babel/dir.js
index 2eaca85750f8..32136b78f02a 100644
--- a/packages/babel-cli/src/babel/dir.js
+++ b/packages/babel-cli/src/babel/dir.js
@@ -1,3 +1,5 @@
+// @flow
+
 import defaults from "lodash/defaults";
 import outputFileSync from "output-file-sync";
 import { sync as mkdirpSync } from "mkdirp";
@@ -6,11 +8,15 @@ import path from "path";
 import fs from "fs";
 
 import * as util from "./util";
+import { type CmdOptions } from "./options";
 
-export default async function({ cliOptions, babelOptions }) {
+export default async function({
+  cliOptions,
+  babelOptions,
+}: CmdOptions): Promise<void> {
   const filenames = cliOptions.filenames;
 
-  async function write(src, base) {
+  async function write(src: string, base: string): Promise<boolean> {
     let relative = path.relative(base, src);
 
     if (!util.isCompilableExtension(relative, cliOptions.extensions)) {
@@ -65,14 +71,14 @@ export default async function({ cliOptions, babelOptions }) {
     }
   }
 
-  function getDest(filename, base) {
+  function getDest(filename: string, base: string): string {
     if (cliOptions.relative) {
       return path.join(base, cliOptions.outDir, filename);
     }
     return path.join(cliOptions.outDir, filename);
   }
 
-  async function handleFile(src, base) {
+  async function handleFile(src: string, base: string): Promise<boolean> {
     const written = await write(src, base);
 
     if (!written && cliOptions.copyFiles) {
@@ -84,7 +90,7 @@ export default async function({ cliOptions, babelOptions }) {
     return written;
   }
 
-  async function handle(filenameOrDir) {
+  async function handle(filenameOrDir: string): Promise<number> {
     if (!fs.existsSync(filenameOrDir)) return 0;
 
     const stat = fs.statSync(filenameOrDir);
diff --git a/packages/babel-cli/src/babel/file.js b/packages/babel-cli/src/babel/file.js
index f0678e27ed39..14cf4c562971 100644
--- a/packages/babel-cli/src/babel/file.js
+++ b/packages/babel-cli/src/babel/file.js
@@ -1,3 +1,5 @@
+// @flow
+
 import convertSourceMap from "convert-source-map";
 import defaults from "lodash/defaults";
 import sourceMap from "source-map";
@@ -6,9 +8,18 @@ import path from "path";
 import fs from "fs";
 
 import * as util from "./util";
-
-export default async function({ cliOptions, babelOptions }) {
-  function buildResult(fileResults) {
+import { type CmdOptions } from "./options";
+
+type CompilationOutput = {
+  code: string,
+  map: Object,
+};
+
+export default async function({
+  cliOptions,
+  babelOptions,
+}: CmdOptions): Promise<void> {
+  function buildResult(fileResults: Array<Object>): CompilationOutput {
     const map = new sourceMap.SourceMapGenerator({
       file:
         cliOptions.sourceMapTarget ||
@@ -74,7 +85,7 @@ export default async function({ cliOptions, babelOptions }) {
     };
   }
 
-  function output(fileResults) {
+  function output(fileResults: Array<string>): void {
     const result = buildResult(fileResults);
 
     if (cliOptions.outFile) {
@@ -91,25 +102,28 @@ export default async function({ cliOptions, babelOptions }) {
     }
   }
 
-  function readStdin() {
-    return new Promise((resolve, reject) => {
-      let code = "";
+  function readStdin(): Promise<string> {
+    return new Promise(
+      (resolve: Function, reject: Function): void => {
+        let code = "";
 
-      process.stdin.setEncoding("utf8");
+        process.stdin.setEncoding("utf8");
 
-      process.stdin.on("readable", function() {
-        const chunk = process.stdin.read();
-        if (chunk !== null) code += chunk;
-      });
+        process.stdin.on("readable", function() {
+          const chunk = process.stdin.read();
+          // $FlowIgnore
+          if (chunk !== null) code += chunk;
+        });
 
-      process.stdin.on("end", function() {
-        resolve(code);
-      });
-      process.stdin.on("error", reject);
-    });
+        process.stdin.on("end", function() {
+          resolve(code);
+        });
+        process.stdin.on("error", reject);
+      },
+    );
   }
 
-  async function stdin() {
+  async function stdin(): Promise<void> {
     const code = await readStdin();
 
     const res = await util.transform(
@@ -126,7 +140,7 @@ export default async function({ cliOptions, babelOptions }) {
     output([res]);
   }
 
-  async function walk(filenames) {
+  async function walk(filenames: Array<string>): Promise<void> {
     const _filenames = [];
 
     filenames.forEach(function(filename) {
@@ -151,7 +165,7 @@ export default async function({ cliOptions, babelOptions }) {
     });
 
     const results = await Promise.all(
-      _filenames.map(async function(filename) {
+      _filenames.map(async function(filename: string): Promise<Object> {
         let sourceFilename = filename;
         if (cliOptions.outFile) {
           sourceFilename = path.relative(
@@ -168,7 +182,7 @@ export default async function({ cliOptions, babelOptions }) {
               {
                 sourceFileName: sourceFilename,
                 // Since we're compiling everything to be merged together,
-                // "inline" applies to the final output file, but to the individual
+                // "inline" applies to the final output file, but not to the individual
                 // files being concatenated.
                 sourceMaps:
                   babelOptions.sourceMaps === "inline"
@@ -192,7 +206,7 @@ export default async function({ cliOptions, babelOptions }) {
     output(results);
   }
 
-  async function files(filenames) {
+  async function files(filenames: Array<string>): Promise<void> {
     if (!cliOptions.skipInitialBuild) {
       await walk(filenames);
     }
@@ -208,7 +222,7 @@ export default async function({ cliOptions, babelOptions }) {
             pollInterval: 10,
           },
         })
-        .on("all", function(type, filename) {
+        .on("all", function(type: string, filename: string) {
           if (!util.isCompilableExtension(filename, cliOptions.extensions)) {
             return;
           }
diff --git a/packages/babel-cli/src/babel/options.js b/packages/babel-cli/src/babel/options.js
index 41cc0fb83e7f..19ae7bfce878 100644
--- a/packages/babel-cli/src/babel/options.js
+++ b/packages/babel-cli/src/babel/options.js
@@ -1,3 +1,5 @@
+// @flow
+
 import fs from "fs";
 
 import commander from "commander";
@@ -151,7 +153,12 @@ commander.option(
 commander.version(pkg.version + " (@babel/core " + version + ")");
 commander.usage("[options] <files ...>");
 
-export default function parseArgv(args: Array<string>) {
+export type CmdOptions = {
+  babelOptions: Object,
+  cliOptions: Object,
+};
+
+export default function parseArgv(args: Array<string>): CmdOptions {
   //
   commander.parse(args);
 
diff --git a/packages/babel-cli/src/babel/util.js b/packages/babel-cli/src/babel/util.js
index 99c9ae9b00cd..daacba53f0d2 100644
--- a/packages/babel-cli/src/babel/util.js
+++ b/packages/babel-cli/src/babel/util.js
@@ -1,10 +1,12 @@
+// @flow
+
 import readdirRecursive from "fs-readdir-recursive";
 import * as babel from "@babel/core";
 import includes from "lodash/includes";
 import path from "path";
 import fs from "fs";
 
-export function chmod(src, dest) {
+export function chmod(src: string, dest: string): void {
   fs.chmodSync(dest, fs.statSync(src).mode);
 }
 
@@ -13,8 +15,8 @@ type ReaddirFilter = (filename: string) => boolean;
 export function readdir(
   dirname: string,
   includeDotfiles: boolean,
-  filter: ReaddirFilter,
-) {
+  filter?: ReaddirFilter,
+): Array<string> {
   return readdirRecursive(dirname, (filename, _index, currentDirectory) => {
     const stat = fs.statSync(path.join(currentDirectory, filename));
 
@@ -30,7 +32,7 @@ export function readdirForCompilable(
   dirname: string,
   includeDotfiles: boolean,
   altExts?: Array<string>,
-) {
+): Array<string> {
   return readdir(dirname, includeDotfiles, function(filename) {
     return isCompilableExtension(filename, altExts);
   });
@@ -48,7 +50,7 @@ export function isCompilableExtension(
   return includes(exts, ext);
 }
 
-export function addSourceMappingUrl(code, loc) {
+export function addSourceMappingUrl(code: string, loc: string): string {
   return code + "\n//# sourceMappingURL=" + path.basename(loc);
 }
 
@@ -56,7 +58,11 @@ const CALLER = {
   name: "@babel/cli",
 };
 
-export function transform(filename, code, opts) {
+export function transform(
+  filename: string,
+  code: string,
+  opts: Object,
+): Promise<Object> {
   opts = {
     ...opts,
     caller: CALLER,
@@ -71,7 +77,10 @@ export function transform(filename, code, opts) {
   });
 }
 
-export function compile(filename, opts) {
+export function compile(
+  filename: string,
+  opts: Object | Function,
+): Promise<Object> {
   opts = {
     ...opts,
     caller: CALLER,
@@ -85,7 +94,7 @@ export function compile(filename, opts) {
   });
 }
 
-export function deleteDir(path) {
+export function deleteDir(path: string): void {
   if (fs.existsSync(path)) {
     fs.readdirSync(path).forEach(function(file) {
       const curPath = path + "/" + file;
@@ -106,7 +115,7 @@ process.on("uncaughtException", function(err) {
   process.exit(1);
 });
 
-export function requireChokidar() {
+export function requireChokidar(): Object {
   try {
     return require("chokidar");
   } catch (err) {
@@ -118,7 +127,10 @@ export function requireChokidar() {
   }
 }
 
-export function adjustRelative(relative, keepFileExtension) {
+export function adjustRelative(
+  relative: string,
+  keepFileExtension: boolean,
+): string {
   if (keepFileExtension) {
     return relative;
   }