diff --git a/.flowconfig b/.flowconfig
index 77b3550cda2b..c003d67d151d 100644
--- a/.flowconfig
+++ b/.flowconfig
@@ -28,3 +28,4 @@ esproposal.export_star_as=enable
 esproposal.optional_chaining=enable
 esproposal.nullish_coalescing=enable
 module.name_mapper='^@babel\/\([a-zA-Z0-9_\-]+\)$' -> '<PROJECT_ROOT>/packages/babel-\1/src/index'
+module.ignore_non_literal_requires=true
diff --git a/lib/third-party-libs.js.flow b/lib/third-party-libs.js.flow
index b41fbe725b4e..f9723ea6fe4e 100644
--- a/lib/third-party-libs.js.flow
+++ b/lib/third-party-libs.js.flow
@@ -2,6 +2,10 @@
  * Basic declarations for the npm modules we use.
  */
 
+declare module "debug" {
+  declare export default (namespace: string) => (formatter: string, ...args: any[]) => void;
+}
+
 declare module "resolve" {
   declare export default {
     (string, {| basedir: string |}, (err: ?Error, res: string) => void): void;
@@ -27,6 +31,10 @@ declare module "lodash/merge" {
   declare export default <T: Object>(T, Object) => T;
 }
 
+declare module "lodash/escapeRegExp" {
+  declare export default (toEscape?: string) => string;
+}
+
 declare module "semver" {
   declare class SemVer {
     build: Array<string>;
diff --git a/packages/babel-core/src/config/caching.js b/packages/babel-core/src/config/caching.js
index 6b6e7967a293..050c59d856dc 100644
--- a/packages/babel-core/src/config/caching.js
+++ b/packages/babel-core/src/config/caching.js
@@ -108,7 +108,7 @@ function makeCachedFunction<ArgT, ResultT, SideChannel, Cache: *>(
     const asyncContext = yield* isAsync();
     const callCache = asyncContext ? callCacheAsync : callCacheSync;
 
-    const cached = yield* getCachedValueOrWait(
+    const cached = yield* getCachedValueOrWait<ArgT, ResultT, SideChannel>(
       asyncContext,
       callCache,
       futureCache,
@@ -119,7 +119,7 @@ function makeCachedFunction<ArgT, ResultT, SideChannel, Cache: *>(
 
     const cache = new CacheConfigurator(data);
 
-    const handlerResult = handler(arg, cache);
+    const handlerResult: Handler<ResultT> | ResultT = handler(arg, cache);
 
     let finishLock: ?Lock<ResultT>;
     let value: ResultT;
@@ -313,7 +313,7 @@ class CacheConfigurator<SideChannel = void> {
     );
 
     if (isThenable(key)) {
-      return key.then(key => {
+      return key.then((key: mixed) => {
         this._pairs.push([key, fn]);
         return key;
       });
@@ -369,7 +369,13 @@ function makeSimpleConfigurator(
 
 // Types are limited here so that in the future these values can be used
 // as part of Babel's caching logic.
-type SimpleType = string | boolean | number | null | void | Promise<SimpleType>;
+export type SimpleType =
+  | string
+  | boolean
+  | number
+  | null
+  | void
+  | Promise<SimpleType>;
 export function assertSimpleType(value: mixed): SimpleType {
   if (isThenable(value)) {
     throw new Error(
diff --git a/packages/babel-core/src/config/config-chain.js b/packages/babel-core/src/config/config-chain.js
index 4fc2f8d0efcf..f54dc4dcd6bd 100644
--- a/packages/babel-core/src/config/config-chain.js
+++ b/packages/babel-core/src/config/config-chain.js
@@ -76,7 +76,6 @@ export const buildPresetChainWalker: (
   arg: PresetInstance,
   context: *,
 ) => * = makeChainWalker({
-  init: arg => arg,
   root: preset => loadPresetDescriptors(preset),
   env: (preset, envName) => loadPresetEnvDescriptors(preset)(envName),
   overrides: (preset, index) => loadPresetOverridesDescriptors(preset)(index),
@@ -419,12 +418,12 @@ function makeChainWalker<ArgT: { options: ValidatedOptions, dirname: string }>({
   env,
   overrides,
   overridesEnv,
-}: {
+}: {|
   root: ArgT => OptionsAndDescriptors,
   env: (ArgT, string) => OptionsAndDescriptors | null,
   overrides: (ArgT, number) => OptionsAndDescriptors,
   overridesEnv: (ArgT, number, string) => OptionsAndDescriptors | null,
-}): (
+|}): (
   ArgT,
   ConfigContext,
   Set<ConfigFile> | void,
diff --git a/packages/babel-core/src/config/files/configuration.js b/packages/babel-core/src/config/files/configuration.js
index f119bc1f8101..887c0334fdf2 100644
--- a/packages/babel-core/src/config/files/configuration.js
+++ b/packages/babel-core/src/config/files/configuration.js
@@ -151,7 +151,7 @@ export function* loadConfig(
  * Read the given config file, returning the result. Returns null if no config was found, but will
  * throw if there are parsing errors while loading a config.
  */
-function readConfig(filepath, envName, caller) {
+function readConfig(filepath, envName, caller): Handler<ConfigFile | null> {
   const ext = path.extname(filepath);
   return ext === ".js" || ext === ".cjs" || ext === ".mjs"
     ? readConfigJS(filepath, { envName, caller })
@@ -236,7 +236,7 @@ const readConfigJS = makeStrongCache(function* readConfigJS(
 
 const packageToBabelConfig = makeWeakCacheSync(
   (file: ConfigFile): ConfigFile | null => {
-    const babel = file.options[("babel": string)];
+    const babel: mixed = file.options[("babel": string)];
 
     if (typeof babel === "undefined") return null;
 
@@ -252,7 +252,7 @@ const packageToBabelConfig = makeWeakCacheSync(
   },
 );
 
-const readConfigJSON5 = makeStaticFileCache((filepath, content) => {
+const readConfigJSON5 = makeStaticFileCache((filepath, content): ConfigFile => {
   let options;
   try {
     options = json5.parse(content);
@@ -281,7 +281,7 @@ const readIgnoreConfig = makeStaticFileCache((filepath, content) => {
   const ignoreDir = path.dirname(filepath);
   const ignorePatterns = content
     .split("\n")
-    .map(line => line.replace(/#(.*?)$/, "").trim())
+    .map<string>(line => line.replace(/#(.*?)$/, "").trim())
     .filter(line => !!line);
 
   for (const pattern of ignorePatterns) {
@@ -299,7 +299,7 @@ const readIgnoreConfig = makeStaticFileCache((filepath, content) => {
   };
 });
 
-function throwConfigError() {
+function throwConfigError(): empty {
   throw new Error(`\
 Caching was left unconfigured. Babel's plugins, presets, and .babelrc.js files can be configured
 for various types of caching, using the first param of their handler functions:
diff --git a/packages/babel-core/src/config/files/import.js b/packages/babel-core/src/config/files/import.js
index e16bc29752a9..d81b18f7539f 100644
--- a/packages/babel-core/src/config/files/import.js
+++ b/packages/babel-core/src/config/files/import.js
@@ -1,3 +1,4 @@
+// @flow
 // We keep this in a seprate file so that in older node versions, where
 // import() isn't supported, we can try/catch around the require() call
 // when loading this file.
diff --git a/packages/babel-core/src/config/files/package.js b/packages/babel-core/src/config/files/package.js
index 278ddfb852f5..99ac13364e77 100644
--- a/packages/babel-core/src/config/files/package.js
+++ b/packages/babel-core/src/config/files/package.js
@@ -39,12 +39,14 @@ const readConfigPackage = makeStaticFileCache(
   (filepath, content): ConfigFile => {
     let options;
     try {
-      options = JSON.parse(content);
+      options = (JSON.parse(content): mixed);
     } catch (err) {
       err.message = `${filepath}: Error while parsing JSON - ${err.message}`;
       throw err;
     }
 
+    if (!options) throw new Error(`${filepath}: No config detected`);
+
     if (typeof options !== "object") {
       throw new Error(`${filepath}: Config returned typeof ${typeof options}`);
     }
diff --git a/packages/babel-core/src/config/files/plugins.js b/packages/babel-core/src/config/files/plugins.js
index be20468d1615..47cfa8645003 100644
--- a/packages/babel-core/src/config/files/plugins.js
+++ b/packages/babel-core/src/config/files/plugins.js
@@ -105,7 +105,7 @@ function resolveStandardizedName(
       try {
         resolve.sync(name, { basedir: dirname });
         resolvedOriginal = true;
-      } catch (e2) {}
+      } catch {}
 
       if (resolvedOriginal) {
         e.message += `\n- If you want to resolve "${name}", use "module:${name}"`;
@@ -118,7 +118,7 @@ function resolveStandardizedName(
         basedir: dirname,
       });
       resolvedBabel = true;
-    } catch (e2) {}
+    } catch {}
 
     if (resolvedBabel) {
       e.message += `\n- Did you mean "@babel/${name}"?`;
@@ -129,7 +129,7 @@ function resolveStandardizedName(
     try {
       resolve.sync(standardizeName(oppositeType, name), { basedir: dirname });
       resolvedOppositeType = true;
-    } catch (e2) {}
+    } catch {}
 
     if (resolvedOppositeType) {
       e.message += `\n- Did you accidentally pass a ${oppositeType} as a ${type}?`;
@@ -151,7 +151,6 @@ function requireModule(type: string, name: string): mixed {
 
   try {
     LOADING_MODULES.add(name);
-    // $FlowIssue
     return require(name);
   } finally {
     LOADING_MODULES.delete(name);
diff --git a/packages/babel-core/src/config/full.js b/packages/babel-core/src/config/full.js
index c9fb038a3074..04fa5b599083 100644
--- a/packages/babel-core/src/config/full.js
+++ b/packages/babel-core/src/config/full.js
@@ -66,7 +66,7 @@ export default gensync<[any], ResolvedConfig | null>(function* loadFullConfig(
   const { options, context } = result;
 
   const optionDefaults = {};
-  const passes = [[]];
+  const passes: Array<Array<Plugin>> = [[]];
   try {
     const { plugins, presets } = options;
 
@@ -74,14 +74,8 @@ export default gensync<[any], ResolvedConfig | null>(function* loadFullConfig(
       throw new Error("Assertion failure - plugins and presets exist");
     }
 
-    const ignored = yield* (function* recurseDescriptors(
-      config: {
-        plugins: Array<UnloadedDescriptor>,
-        presets: Array<UnloadedDescriptor>,
-      },
-      pass: Array<Plugin>,
-    ) {
-      const plugins = [];
+    const ignored = yield* (function* recurseDescriptors(config, pass) {
+      const plugins: Array<Plugin> = [];
       for (let i = 0; i < config.plugins.length; i++) {
         const descriptor = config.plugins[i];
         if (descriptor.options !== false) {
@@ -103,7 +97,10 @@ export default gensync<[any], ResolvedConfig | null>(function* loadFullConfig(
         }
       }
 
-      const presets = [];
+      const presets: Array<{|
+        preset: ConfigChain | null,
+        pass: Array<Plugin>,
+      |}> = [];
       for (let i = 0; i < config.presets.length; i++) {
         const descriptor = config.presets[i];
         if (descriptor.options !== false) {
diff --git a/packages/babel-core/src/config/helpers/config-api.js b/packages/babel-core/src/config/helpers/config-api.js
index f49e038f764e..fea73139af68 100644
--- a/packages/babel-core/src/config/helpers/config-api.js
+++ b/packages/babel-core/src/config/helpers/config-api.js
@@ -6,6 +6,7 @@ import {
   assertSimpleType,
   type CacheConfigurator,
   type SimpleCacheConfigurator,
+  type SimpleType,
 } from "../caching";
 
 import type { CallerMetadata } from "../validation/options";
@@ -17,13 +18,15 @@ type EnvFunction = {
   (Array<string>): boolean,
 };
 
+type CallerFactory = ((CallerMetadata | void) => mixed) => SimpleType;
+
 export type PluginAPI = {|
   version: string,
   cache: SimpleCacheConfigurator,
   env: EnvFunction,
   async: () => boolean,
   assertVersion: typeof assertVersion,
-  caller?: any,
+  caller?: CallerFactory,
 |};
 
 export default function makeAPI(
@@ -37,7 +40,7 @@ export default function makeAPI(
       }
       if (!Array.isArray(value)) value = [value];
 
-      return value.some(entry => {
+      return value.some((entry: mixed) => {
         if (typeof entry !== "string") {
           throw new Error("Unexpected non-string value");
         }
@@ -45,8 +48,7 @@ export default function makeAPI(
       });
     });
 
-  const caller: any = cb =>
-    cache.using(data => assertSimpleType(cb(data.caller)));
+  const caller = cb => cache.using(data => assertSimpleType(cb(data.caller)));
 
   return {
     version: coreVersion,
diff --git a/packages/babel-core/src/config/item.js b/packages/babel-core/src/config/item.js
index c847df029e52..d1f548ffe0fa 100644
--- a/packages/babel-core/src/config/item.js
+++ b/packages/babel-core/src/config/item.js
@@ -103,7 +103,7 @@ class ConfigItem {
     // programmatically, and also make sure that if people happen to
     // pass the item through JSON.stringify, it doesn't show up.
     this._descriptor = descriptor;
-    Object.defineProperty(this, "_descriptor", ({ enumerable: false }: any));
+    Object.defineProperty(this, "_descriptor", { enumerable: false });
 
     this.value = this._descriptor.value;
     this.options = this._descriptor.options;
diff --git a/packages/babel-core/src/config/plugin.js b/packages/babel-core/src/config/plugin.js
index 6da0fe636014..c034a0976c31 100644
--- a/packages/babel-core/src/config/plugin.js
+++ b/packages/babel-core/src/config/plugin.js
@@ -4,7 +4,7 @@ import type { PluginObject } from "./validation/plugins";
 
 export default class Plugin {
   key: ?string;
-  manipulateOptions: Function | void;
+  manipulateOptions: ((options: mixed, parserOpts: mixed) => void) | void;
   post: Function | void;
   pre: Function | void;
   visitor: {};
diff --git a/packages/babel-core/src/config/validation/option-assertions.js b/packages/babel-core/src/config/validation/option-assertions.js
index a279a72781a0..f3962f18945f 100644
--- a/packages/babel-core/src/config/validation/option-assertions.js
+++ b/packages/babel-core/src/config/validation/option-assertions.js
@@ -18,6 +18,8 @@ import type {
   RootMode,
 } from "./options";
 
+export type { RootPath } from "./options";
+
 export type ValidatorSet = {
   [string]: Validator<any>,
 };
@@ -192,7 +194,10 @@ export function assertBoolean(loc: GeneralPath, value: mixed): boolean | void {
   return value;
 }
 
-export function assertObject(loc: GeneralPath, value: mixed): {} | void {
+export function assertObject(
+  loc: GeneralPath,
+  value: mixed,
+): { +[string]: mixed } | void {
   if (
     value !== undefined &&
     (typeof value !== "object" || Array.isArray(value) || !value)
diff --git a/packages/babel-core/src/config/validation/options.js b/packages/babel-core/src/config/validation/options.js
index c3e7f359cd90..d4500a747c26 100644
--- a/packages/babel-core/src/config/validation/options.js
+++ b/packages/babel-core/src/config/validation/options.js
@@ -276,7 +276,7 @@ export type OptionsSource =
   | "preset"
   | "plugin";
 
-type RootPath = $ReadOnly<{
+export type RootPath = $ReadOnly<{
   type: "root",
   source: OptionsSource,
 }>;
@@ -311,7 +311,7 @@ function validateNested(loc: NestingPath, opts: {}) {
 
   assertNoDuplicateSourcemap(opts);
 
-  Object.keys(opts).forEach(key => {
+  Object.keys(opts).forEach((key: string) => {
     const optLoc = {
       type: "option",
       name: key,
@@ -364,7 +364,10 @@ function throwUnknownError(loc: OptionPath) {
   const key = loc.name;
 
   if (removed[key]) {
-    const { message, version = 5 } = removed[key];
+    const {
+      message,
+      version = 5,
+    }: { message: string, version?: number } = removed[key];
 
     throw new Error(
       `Using removed Babel ${version} option: ${msg(loc)} - ${message}`,
diff --git a/packages/babel-core/src/config/validation/plugins.js b/packages/babel-core/src/config/validation/plugins.js
index 06778caff269..63510dd13c25 100644
--- a/packages/babel-core/src/config/validation/plugins.js
+++ b/packages/babel-core/src/config/validation/plugins.js
@@ -1,9 +1,13 @@
+// @flow
 import {
   assertString,
   assertFunction,
   assertObject,
+  msg,
   type ValidatorSet,
   type Validator,
+  type OptionPath,
+  type RootPath,
 } from "./option-assertions";
 
 // Note: The casts here are just meant to be static assertions to make sure
@@ -31,14 +35,16 @@ const VALIDATORS: ValidatorSet = {
   >),
 };
 
-function assertVisitorMap(key: string, value: mixed): VisitorMap {
-  const obj = assertObject(key, value);
+function assertVisitorMap(loc: OptionPath, value: mixed): VisitorMap {
+  const obj = assertObject(loc, value);
   if (obj) {
     Object.keys(obj).forEach(prop => assertVisitorHandler(prop, obj[prop]));
 
     if (obj.enter || obj.exit) {
       throw new Error(
-        `.${key} cannot contain catch-all "enter" or "exit" handlers. Please target individual nodes.`,
+        `${msg(
+          loc,
+        )} cannot contain catch-all "enter" or "exit" handlers. Please target individual nodes.`,
       );
     }
   }
@@ -50,7 +56,7 @@ function assertVisitorHandler(
   value: mixed,
 ): VisitorHandler | void {
   if (value && typeof value === "object") {
-    Object.keys(value).forEach(handler => {
+    Object.keys(value).forEach((handler: string) => {
       if (handler !== "enter" && handler !== "exit") {
         throw new Error(
           `.visitor["${key}"] may only have .enter and/or .exit handlers.`,
@@ -71,7 +77,7 @@ export type VisitorMap = {
 
 export type PluginObject = {
   name?: string,
-  manipulateOptions?: Function,
+  manipulateOptions?: (options: mixed, parserOpts: mixed) => void,
 
   pre?: Function,
   post?: Function,
@@ -88,16 +94,17 @@ export function validatePluginObject(obj: {}): PluginObject {
     type: "root",
     source: "plugin",
   };
-  Object.keys(obj).forEach(key => {
+  Object.keys(obj).forEach((key: string) => {
     const validator = VALIDATORS[key];
-    const optLoc = {
-      type: "option",
-      name: key,
-      parent: rootPath,
-    };
 
-    if (validator) validator(optLoc, obj[key]);
-    else {
+    if (validator) {
+      const optLoc: OptionPath = {
+        type: "option",
+        name: key,
+        parent: rootPath,
+      };
+      validator(optLoc, obj[key]);
+    } else {
       const invalidPluginPropertyError = new Error(
         `.${key} is not a valid Plugin property`,
       );
diff --git a/packages/babel-core/test/config-loading.js b/packages/babel-core/test/config-loading.js
index d62a948254cf..736534ecd8ae 100644
--- a/packages/babel-core/test/config-loading.js
+++ b/packages/babel-core/test/config-loading.js
@@ -339,6 +339,23 @@ describe("@babel/core config loading", () => {
         /\.inherits must be a function, or undefined/,
       );
     });
+
+    it("should throw when plugin contains `enter` handler", () => {
+      const fooPlugin = {
+        visitor: {
+          enter() {},
+        },
+      };
+      const opts = {
+        cwd: path.dirname(FILEPATH),
+        filename: FILEPATH,
+        plugins: [fooPlugin],
+      };
+
+      expect(() => loadConfig(opts)).toThrow(
+        /\.visitor cannot contain catch-all "enter" or "exit" handlers\. Please target individual nodes\./,
+      );
+    });
   });
 
   describe("caller metadata", () => {