diff --git a/package.json b/package.json
index 87c7a2c49..200aa2a99 100644
--- a/package.json
+++ b/package.json
@@ -47,6 +47,7 @@
     "babel-plugin-macros": "^2.6.1",
     "babel-plugin-transform-async-to-promises": "^0.8.14",
     "babel-plugin-transform-rename-import": "^2.3.0",
+    "babel-plugin-transform-replace-expressions": "^0.2.0",
     "babel-traverse": "^6.26.0",
     "babylon": "^6.18.0",
     "camelcase": "^5.0.0",
diff --git a/src/createRollupConfig.ts b/src/createRollupConfig.ts
index 816682e0c..ff887b0a1 100644
--- a/src/createRollupConfig.ts
+++ b/src/createRollupConfig.ts
@@ -1,8 +1,14 @@
-import { safeVariableName, safePackageName, external } from './utils';
+import {
+  safeVariableName,
+  safePackageName,
+  external,
+  toReplacementExpression,
+  parseMappingArgument,
+} from './utils';
 import { paths } from './constants';
 import { terser } from 'rollup-plugin-terser';
 import { DEFAULT_EXTENSIONS } from '@babel/core';
-// import babel from 'rollup-plugin-babel';
+import babel from 'rollup-plugin-babel';
 import commonjs from 'rollup-plugin-commonjs';
 import json from 'rollup-plugin-json';
 import replace from 'rollup-plugin-replace';
@@ -39,6 +45,13 @@ export function createRollupConfig(opts: TsdxOptions) {
     .filter(Boolean)
     .join('.');
 
+  let defines = {};
+  if (opts.define) {
+    defines = Object.assign(
+      defines,
+      parseMappingArgument(opts.define, toReplacementExpression)
+    );
+  }
   return {
     // Tell Rollup the entry point to the package
     input: opts.input,
@@ -142,6 +155,20 @@ export function createRollupConfig(opts: TsdxOptions) {
           },
         },
       }),
+      // if defines is not set, we shouldn't run babel through node_modules
+      !!defines &&
+        babel({
+          babelrc: false,
+          configFile: false,
+          compact: false,
+          include: 'node_modules/**',
+          plugins: [
+            [
+              require.resolve('babel-plugin-transform-replace-expressions'),
+              { replace: defines },
+            ],
+          ],
+        }),
       babelPluginTsdx({
         exclude: 'node_modules/**',
         extensions: [...DEFAULT_EXTENSIONS, 'ts', 'tsx'],
@@ -150,7 +177,7 @@ export function createRollupConfig(opts: TsdxOptions) {
           targets: opts.target === 'node' ? { node: '8' } : undefined,
           extractErrors: opts.extractErrors,
           format: opts.format,
-          // defines: opts.defines,
+          defines,
         },
       }),
       opts.env !== undefined &&
@@ -174,6 +201,6 @@ export function createRollupConfig(opts: TsdxOptions) {
           toplevel: opts.format === 'cjs',
           warnings: true,
         }),
-    ],
+    ].filter(Boolean),
   };
 }
diff --git a/src/index.ts b/src/index.ts
index 53eb7b076..56e87a2c0 100755
--- a/src/index.ts
+++ b/src/index.ts
@@ -316,8 +316,10 @@ prog
   .example('watch --verbose')
   .option('--tsconfig', 'Specify custom tsconfig path')
   .example('watch --tsconfig ./tsconfig.foo.json')
-  .option('--extractErrors', 'Extract invariant errors to ./errors/codes.json.')
-  .example('build --extractErrors')
+  .option('--extractErrors', 'Extract invariant errors to ./errors/codes.json')
+  .example('watch --extractErrors')
+  .option('--define', 'Replace constants with hard-coded values')
+  .example('watch --define A=1,ZOOP=FOO')
   .action(async (dirtyOpts: any) => {
     const opts = await normalizeOpts(dirtyOpts);
     const buildConfigs = createBuildConfigs(opts);
@@ -375,13 +377,10 @@ prog
   .example('build --format cjs,esm')
   .option('--tsconfig', 'Specify custom tsconfig path')
   .example('build --tsconfig ./tsconfig.foo.json')
-  .option(
-    '--extractErrors',
-    'Extract errors to ./errors/codes.json and provide a url for decoding.'
-  )
-  .example(
-    'build --extractErrors=https://reactjs.org/docs/error-decoder.html?invariant='
-  )
+  .option('--extractErrors', 'Extract errors to ./errors/codes.json')
+  .example('build --extractErrors')
+  .option('--define', 'Replace constants with hard-coded values')
+  .example('build --define A=1,ZOOP=FOO')
   .action(async (dirtyOpts: any) => {
     const opts = await normalizeOpts(dirtyOpts);
     const buildConfigs = createBuildConfigs(opts);
diff --git a/src/types.ts b/src/types.ts
index 907ea4550..e2acd5fe0 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -17,4 +17,6 @@ export interface TsdxOptions {
   minify?: boolean;
   // Is this the very first rollup config (and thus should one-off metadata be extracted)?
   writeMeta?: boolean;
+  // Define key values to be replaced (e.g. --define A=1)
+  define?: string;
 }
diff --git a/src/utils.ts b/src/utils.ts
index 85385a83e..241395979 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -35,3 +35,44 @@ export function clearConsole() {
     process.platform === 'win32' ? '\x1B[2J\x1B[0f' : '\x1B[2J\x1B[3J\x1B[H'
   );
 }
+
+// Convert booleans and int define= values to literals.
+// This is more intuitive than `microbundle --define A=1` producing A="1".
+export const toReplacementExpression = (value: string, name: string) => {
+  // --define A="1",B='true' produces string:
+  const matches = value.match(/^(['"])(.+)\1$/);
+  if (matches) {
+    return [JSON.stringify(matches[2]), name];
+  }
+
+  // --define A=1,B=true produces int/boolean literal:
+  if (/^(true|false|\d+)$/i.test(value)) {
+    return [value, name];
+  }
+
+  // default: string literal
+  return [JSON.stringify(value), name];
+};
+
+// Parses values of the form "$=jQuery,React=react" into key-value object pairs.
+export const parseMappingArgument = (
+  globalStrings: string,
+  processValue: (value: string, name: string) => any
+) => {
+  const globals: any = {};
+  globalStrings.split(',').forEach(globalString => {
+    let [key, value] = globalString.split('=');
+    if (processValue) {
+      const r = processValue(value, key);
+      if (r !== undefined) {
+        if (Array.isArray(r)) {
+          [value, key] = r;
+        } else {
+          value = r;
+        }
+      }
+    }
+    globals[key] = value;
+  });
+  return globals;
+};
diff --git a/yarn.lock b/yarn.lock
index 888197e75..db0c5091e 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -253,7 +253,7 @@
   resolved "https://registry.yarnpkg.com/@babel/parser/-/parser-7.2.3.tgz#32f5df65744b70888d17872ec106b02434ba1489"
   integrity sha512-0LyEcVlfCoFmci8mXx8A5oIkpkOgyo8dRHtxBnK9RRBwxO2+JZPNsqtVEZQ7mJFPxnXF9lfmU24mHOPI0qnlkA==
 
-"@babel/parser@^7.4.5", "@babel/parser@^7.5.5":
+"@babel/parser@^7.3.3", "@babel/parser@^7.4.5", "@babel/parser@^7.5.5":
   version "7.5.5"
   resolved "https://registry.yarnpkg.com/@babel/parser/-/parser-7.5.5.tgz#02f077ac8817d3df4a832ef59de67565e71cca4b"
   integrity sha512-E5BN68cqR7dhKan1SfqgPGhQ178bkVKpXTPEXnFJBrEt8/DKRZlybmy+IgYLTeN7tp1R5Ccmbm2rBk17sHYU3g==
@@ -1689,6 +1689,13 @@ babel-plugin-transform-rename-import@^2.3.0:
   resolved "https://registry.yarnpkg.com/babel-plugin-transform-rename-import/-/babel-plugin-transform-rename-import-2.3.0.tgz#5d9d645f937b0ca5c26a24b2510a06277b6ffd9b"
   integrity sha512-dPgJoT57XC0PqSnLgl2FwNvxFrWlspatX2dkk7yjKQj5HHGw071vAcOf+hqW8ClqcBDMvEbm6mevn5yHAD8mlQ==
 
+babel-plugin-transform-replace-expressions@^0.2.0:
+  version "0.2.0"
+  resolved "https://registry.yarnpkg.com/babel-plugin-transform-replace-expressions/-/babel-plugin-transform-replace-expressions-0.2.0.tgz#59cba8df4b4a675e7c78cd21548f8e7685bbc30d"
+  integrity sha512-Eh1rRd9hWEYgkgoA3D0kGp7xJ/wgVshgsqmq60iC4HVWD+Lux+fNHSHBa2v1Hsv+dHflShC71qKhiH40OiPtDA==
+  dependencies:
+    "@babel/parser" "^7.3.3"
+
 babel-preset-jest@^24.6.0:
   version "24.6.0"
   resolved "https://registry.yarnpkg.com/babel-preset-jest/-/babel-preset-jest-24.6.0.tgz#66f06136eefce87797539c0d63f1769cc3915984"