diff --git a/index.d.ts b/index.d.ts
index df3f10e..56db00d 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -32,7 +32,7 @@ declare namespace meow {
 		/**
 		Define argument flags.
 
-		The key is the flag name and the value is an object with any of:
+		The key is the flag name in camel-case and the value is an object with any of:
 
 		- `type`: Type of value. (Possible values: `string` `boolean` `number`)
 		- `alias`: Usually used to define a short flag alias.
@@ -42,6 +42,8 @@ declare namespace meow {
 		- `isMultiple`: Indicates a flag can be set multiple times. Values are turned into an array. (Default: false)
 			Multiple values are provided by specifying the flag multiple times, for example, `$ foo -u rainbow -u cat`. Space- or comma-separated values are *not* supported.
 
+		Note that flags are always defined using a camel-case key (`myKey`), but will match arguments in kebab-case (`--my-key`).
+
 		@example
 		```
 		flags: {
diff --git a/index.js b/index.js
index dccbcb5..75994fa 100644
--- a/index.js
+++ b/index.js
@@ -3,6 +3,7 @@ const path = require('path');
 const buildParserOptions = require('minimist-options');
 const parseArguments = require('yargs-parser');
 const camelCaseKeys = require('camelcase-keys');
+const decamelize = require('decamelize');
 const decamelizeKeys = require('decamelize-keys');
 const trimNewlines = require('trim-newlines');
 const redent = require('redent');
@@ -50,7 +51,14 @@ const getMissingRequiredFlags = (flags, receivedFlags, input) => {
 const reportMissingRequiredFlags = missingRequiredFlags => {
 	console.error(`Missing required flag${missingRequiredFlags.length > 1 ? 's' : ''}`);
 	for (const flag of missingRequiredFlags) {
-		console.error(`\t--${flag.key}${flag.alias ? `, -${flag.alias}` : ''}`);
+		console.error(`\t--${decamelize(flag.key, '-')}${flag.alias ? `, -${flag.alias}` : ''}`);
+	}
+};
+
+const validateOptions = ({flags}) => {
+	const invalidFlags = Object.keys(flags).filter(flagKey => flagKey.includes('-') && flagKey !== '--');
+	if (invalidFlags.length > 0) {
+		throw new Error(`Flag keys may not contain '-': ${invalidFlags.join(', ')}`);
 	}
 };
 
@@ -117,6 +125,7 @@ const meow = (helpText, options) => {
 		hardRejection();
 	}
 
+	validateOptions(options);
 	let parserOptions = {
 		arguments: options.input,
 		...buildParserFlags(options)
diff --git a/package.json b/package.json
index 89485ca..fd367ac 100644
--- a/package.json
+++ b/package.json
@@ -42,6 +42,7 @@
 	"dependencies": {
 		"@types/minimist": "^1.2.0",
 		"camelcase-keys": "^6.2.2",
+		"decamelize": "^1.2.0",
 		"decamelize-keys": "^1.1.0",
 		"hard-rejection": "^2.1.0",
 		"minimist-options": "4.1.0",
diff --git a/readme.md b/readme.md
index 1c547dc..b395915 100644
--- a/readme.md
+++ b/readme.md
@@ -132,7 +132,7 @@ Type: `object`
 
 Define argument flags.
 
-The key is the flag name and the value is an object with any of:
+The key is the flag name in camel-case and the value is an object with any of:
 
 - `type`: Type of value. (Possible values: `string` `boolean` `number`)
 - `alias`: Usually used to define a short flag alias.
@@ -145,6 +145,8 @@ The key is the flag name and the value is an object with any of:
 - `isMultiple`: Indicates a flag can be set multiple times. Values are turned into an array. (Default: false)
 	- Multiple values are provided by specifying the flag multiple times, for example, `$ foo -u rainbow -u cat`. Space- or comma-separated values are [currently *not* supported](https://github.com/sindresorhus/meow/issues/164).
 
+Note that flags are always defined using a camel-case key (`myKey`), but will match arguments in kebab-case (`--my-key`).
+
 Example:
 
 ```js
diff --git a/test/fixtures/fixture-required.js b/test/fixtures/fixture-required.js
index e34577b..2f3690e 100755
--- a/test/fixtures/fixture-required.js
+++ b/test/fixtures/fixture-required.js
@@ -18,6 +18,10 @@ const cli = meow({
 			type: 'number',
 			isRequired: true
 		},
+		kebabCase: {
+			type: 'string',
+			isRequired: true
+		},
 		notRequired: {
 			type: 'string'
 		}
diff --git a/test/is-required-flag.js b/test/is-required-flag.js
index a8c5730..4586b28 100644
--- a/test/is-required-flag.js
+++ b/test/is-required-flag.js
@@ -15,7 +15,8 @@ test('spawn cli and test not specifying required flags', async t => {
 		t.regex(stderr, /Missing required flag/);
 		t.regex(stderr, /--test, -t/);
 		t.regex(stderr, /--number/);
-		t.notRegex(stderr, /--notRequired/);
+		t.regex(stderr, /--kebab-case/);
+		t.notRegex(stderr, /--not-required/);
 	}
 });
 
@@ -24,7 +25,9 @@ test('spawn cli and test specifying all required flags', async t => {
 		'-t',
 		'test',
 		'--number',
-		'6'
+		'6',
+		'--kebab-case',
+		'test'
 	]);
 	t.is(stdout, 'test,6');
 });
@@ -63,7 +66,7 @@ test('spawn cli and test setting isRequired as a function and specifying only th
 		const {stderr, message} = error;
 		t.regex(message, /Command failed with exit code 2/);
 		t.regex(stderr, /Missing required flag/);
-		t.regex(stderr, /--withTrigger/);
+		t.regex(stderr, /--with-trigger/);
 	}
 });
 
diff --git a/test/test.js b/test/test.js
index cfae265..c8dc19d 100644
--- a/test/test.js
+++ b/test/test.js
@@ -99,6 +99,13 @@ test('single character flag casing should be preserved', t => {
 	t.deepEqual(meow({argv: ['-F']}).flags, {F: true});
 });
 
+test('flag declared in kebab-case is an error', t => {
+	const error = t.throws(() => {
+		meow({flags: {'kebab-case': 'boolean', test: 'boolean', 'another-one': 'boolean'}});
+	});
+	t.is(error.message, 'Flag keys may not contain \'-\': kebab-case, another-one');
+});
+
 test('type inference', t => {
 	t.is(meow({argv: ['5']}).input[0], '5');
 	t.is(meow({argv: ['5']}, {input: 'string'}).input[0], '5');