diff --git a/packages/docusaurus/src/commands/start.ts b/packages/docusaurus/src/commands/start.ts
index 6bc6e78ad0..568a777996 100644
--- a/packages/docusaurus/src/commands/start.ts
+++ b/packages/docusaurus/src/commands/start.ts
@@ -59,7 +59,7 @@ export async function start(
   };
 
   const pluginPaths = _.compact(
-    _.flatten(
+    _.flatten<string | undefined>(
       plugins.map(plugin => plugin.getPathsToWatch && plugin.getPathsToWatch()),
     ),
   ).map(normalizeToSiteDir);
diff --git a/packages/docusaurus/src/server/config.ts b/packages/docusaurus/src/server/config.ts
index 4061330b05..5f755b3488 100644
--- a/packages/docusaurus/src/server/config.ts
+++ b/packages/docusaurus/src/server/config.ts
@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import {PluginConfig} from './plugins';
+
 import fs from 'fs-extra';
 import _ from 'lodash';
 import importFresh from 'import-fresh';
@@ -21,7 +23,7 @@ export interface DocusaurusConfig {
   projectName?: string;
   customFields?: string[];
   githubHost?: string;
-  plugins?: any[];
+  plugins?: PluginConfig[];
   presets?: any[];
   themeConfig?: {
     [key: string]: any;
diff --git a/packages/docusaurus/src/server/index.ts b/packages/docusaurus/src/server/index.ts
index 6539501f10..a834fc4a03 100644
--- a/packages/docusaurus/src/server/index.ts
+++ b/packages/docusaurus/src/server/index.ts
@@ -5,7 +5,11 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import {PluginConfig, Plugin} from './plugins';
+
 import path from 'path';
+import _ from 'lodash';
+
 import {generate} from '@docusaurus/utils';
 
 import {loadConfig, DocusaurusConfig} from './config';
@@ -29,7 +33,7 @@ export interface LoadContext {
 }
 export interface Props extends LoadContext {
   routesPaths: string[];
-  plugins: any[];
+  plugins: Plugin<any>[];
 }
 
 export async function load(
@@ -60,11 +64,11 @@ export async function load(
     baseUrl,
   };
 
-  /* Preset */
+  // Presets.
   const {plugins: presetPlugins, themes: presetThemes} = loadPresets(context);
 
-  /* Plugin */
-  const pluginConfigs = [
+  // Plugins.
+  const pluginConfigs: PluginConfig[] = [
     ...presetPlugins,
     ...(siteConfig.plugins || []),
     ...presetThemes,
@@ -75,15 +79,16 @@ export async function load(
     context,
   });
 
-  /* Theme */
+  // Themes.
   const fallbackTheme = path.resolve(__dirname, '../client/theme-fallback');
-  const pluginThemes = plugins
-    .map(plugin => plugin.getThemePath && plugin.getThemePath())
-    .filter(Boolean) as string[];
+  const pluginThemes = _.compact(
+    plugins.map(plugin => plugin.getThemePath && plugin.getThemePath()),
+  );
   const userTheme = path.resolve(siteDir, 'theme');
   const alias = loadThemeAlias([fallbackTheme, ...pluginThemes, userTheme]);
   // Make a fake plugin to resolve aliased theme components.
   plugins.push({
+    name: 'docusaurus-bootstrap-plugin',
     configureWebpack: () => ({
       resolve: {
         alias,
diff --git a/packages/docusaurus/src/server/plugins/index.ts b/packages/docusaurus/src/server/plugins/index.ts
index 334ab55146..d3e99381fc 100644
--- a/packages/docusaurus/src/server/plugins/index.ts
+++ b/packages/docusaurus/src/server/plugins/index.ts
@@ -5,21 +5,51 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import {LoadContext, Props} from '..';
+import {RouteConfig} from '../routes';
+
 import fs from 'fs-extra';
 import importFresh from 'import-fresh';
 import path from 'path';
 import {generate} from '@docusaurus/utils';
-import {LoadContext} from '..';
+import {Configuration} from 'webpack';
+
+export interface Plugin<T> {
+  name: string;
+  loadContent?(): T;
+  contentLoaded?({
+    content: T,
+    actions: DocusaurusPluginContentLoadedActions,
+  }): void;
+  postBuild?(props: Props): void;
+  postStart?(props: Props): void;
+  configureWebpack?(config: Configuration, isServer: boolean): Configuration;
+  getThemePath?(): string;
+  getPathsToWatch?(): string[];
+}
+
+export interface PluginConfig {
+  module: string;
+  options?: Object;
+}
+
+export interface PluginContentLoadedActions {
+  addRoute(config: RouteConfig): void;
+  createData(name: string, data: Object): Promise<string>;
+}
 
 export async function loadPlugins({
-  pluginConfigs = [],
+  pluginConfigs,
   context,
 }: {
-  pluginConfigs: any[];
+  pluginConfigs: PluginConfig[];
   context: LoadContext;
-}) {
+}): Promise<{
+  plugins: Plugin<any>[];
+  pluginsRouteConfigs: RouteConfig[];
+}> {
   // 1. Plugin Lifecycle - Initialization/Constructor
-  const plugins = pluginConfigs.map(({module, options}) => {
+  const plugins: Plugin<any>[] = pluginConfigs.map(({module, options}) => {
     // module is any valid module identifier - npm package or locally-resolved path.
     const plugin = importFresh(module);
     return plugin(context, options);
@@ -40,7 +70,7 @@ export async function loadPlugins({
   );
 
   // 3. Plugin lifecycle - contentLoaded
-  const pluginsRouteConfigs: any[] = [];
+  const pluginsRouteConfigs: RouteConfig[] = [];
 
   await Promise.all(
     plugins.map(async (plugin, index) => {
@@ -52,7 +82,8 @@ export async function loadPlugins({
         context.generatedFilesDir,
         plugin.name,
       );
-      const actions = {
+
+      const actions: PluginContentLoadedActions = {
         addRoute: config => pluginsRouteConfigs.push(config),
         createData: async (name, content) => {
           const modulePath = path.join(pluginContentDir, name);
@@ -62,9 +93,8 @@ export async function loadPlugins({
         },
       };
 
-      const loadedContent = pluginsLoadedContent[index];
       await plugin.contentLoaded({
-        content: loadedContent,
+        content: pluginsLoadedContent[index],
         actions,
       });
     }),
diff --git a/packages/docusaurus/src/server/presets/index.ts b/packages/docusaurus/src/server/presets/index.ts
index 49c27034b6..84c67ed983 100644
--- a/packages/docusaurus/src/server/presets/index.ts
+++ b/packages/docusaurus/src/server/presets/index.ts
@@ -5,31 +5,45 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+import {LoadContext} from '../index';
+import {PluginConfig} from '../plugins';
+
 import importFresh from 'import-fresh';
 import _ from 'lodash';
-import {LoadContext} from '../index';
 
-export function loadPresets(context: LoadContext) {
+export interface Preset {
+  plugins?: PluginConfig[];
+  themes?: PluginConfig[];
+}
+
+export function loadPresets(
+  context: LoadContext,
+): {
+  plugins: PluginConfig[];
+  themes: PluginConfig[];
+} {
   const presets: any[] = context.siteConfig.presets || [];
-  const plugins: any[] = [];
-  const themes: any[] = [];
+  const plugins: (PluginConfig[] | undefined)[] = [];
+  const themes: (PluginConfig[] | undefined)[] = [];
 
   presets.forEach(presetItem => {
-    let presetModule;
+    let presetModuleImport;
     let presetOptions = {};
     if (typeof presetItem === 'string') {
-      presetModule = presetItem;
+      presetModuleImport = presetItem;
     } else if (Array.isArray(presetItem)) {
-      [presetModule, presetOptions] = presetItem;
+      [presetModuleImport, presetOptions] = presetItem;
     }
 
-    const preset = importFresh(presetModule);
-    plugins.push(preset(context, presetOptions).plugins);
-    themes.push(preset(context, presetOptions).themes);
+    const presetModule = importFresh(presetModuleImport);
+    const preset: Preset = presetModule(context, presetOptions);
+
+    plugins.push(preset.plugins);
+    themes.push(preset.themes);
   });
 
   return {
-    plugins: _.compact(_.flatten(plugins)),
-    themes: _.compact(_.flatten(themes)),
+    plugins: _.compact(_.flatten<PluginConfig | undefined>(plugins)),
+    themes: _.compact(_.flatten<PluginConfig | undefined>(themes)),
   };
 }
diff --git a/packages/docusaurus/src/server/themes/alias.ts b/packages/docusaurus/src/server/themes/alias.ts
index 2435d90cb7..1cdb5635f7 100644
--- a/packages/docusaurus/src/server/themes/alias.ts
+++ b/packages/docusaurus/src/server/themes/alias.ts
@@ -10,11 +10,11 @@ import fs from 'fs-extra';
 import path from 'path';
 import {fileToPath, posixPath, normalizeUrl} from '@docusaurus/utils';
 
-export interface Alias {
+export interface ThemeAlias {
   [alias: string]: string;
 }
 
-export function themeAlias(themePath: string): Alias {
+export function themeAlias(themePath: string): ThemeAlias {
   if (!fs.pathExistsSync(themePath)) {
     return {};
   }
@@ -23,7 +23,7 @@ export function themeAlias(themePath: string): Alias {
     cwd: themePath,
   });
 
-  const alias = {};
+  const alias: ThemeAlias = {};
   themeComponentFiles.forEach(relativeSource => {
     const filePath = path.join(themePath, relativeSource);
     const fileName = fileToPath(relativeSource);
diff --git a/packages/docusaurus/src/server/themes/index.ts b/packages/docusaurus/src/server/themes/index.ts
index 553ec1cfb2..5b7f081386 100644
--- a/packages/docusaurus/src/server/themes/index.ts
+++ b/packages/docusaurus/src/server/themes/index.ts
@@ -5,11 +5,11 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-import {themeAlias, Alias} from './alias';
+import {themeAlias, ThemeAlias} from './alias';
 
-export {Alias} from './alias';
+export {ThemeAlias} from './alias';
 
-export function loadThemeAlias(themePaths: string[]): Alias {
+export function loadThemeAlias(themePaths: string[]): ThemeAlias {
   return themePaths.reduce(
     (alias, themePath) => ({
       ...alias,