diff --git a/css-audit.config.js b/css-audit.config.js
index d6dd8d8..3de1d2c 100644
--- a/css-audit.config.js
+++ b/css-audit.config.js
@@ -2,6 +2,7 @@ module.exports = {
 	format: 'html',
 	filename: 'wp-admin',
 	audits: [
+		'custom-props',
 		'colors',
 		'important',
 		'display-none',
diff --git a/src/__tests__/custom-properties.js b/src/__tests__/custom-properties.js
new file mode 100644
index 0000000..d16db2a
--- /dev/null
+++ b/src/__tests__/custom-properties.js
@@ -0,0 +1,67 @@
+const audit = require( '../audits/custom-properties' );
+
+function getResultValue( results, key ) {
+	const { value } = results.find( ( { id } ) => key === id );
+	return value;
+}
+
+describe( 'Audit: Custom Properties', () => {
+	it( 'should return no values when no custom properties', () => {
+		const files = [
+			{
+				name: 'a.css',
+				content: `body { font-size: 1em; line-height: 1.6; color: red }`,
+			},
+		];
+		const { results } = audit( files );
+		expect( getResultValue( results, 'unused' ) ).toHaveLength( 0 );
+		expect( getResultValue( results, 'undefined' ) ).toHaveLength( 0 );
+	} );
+
+	it( 'should count the number of custom properties in a file', () => {
+		const files = [
+			{
+				name: 'a.css',
+				content: `:root { --foo: red; --bar: blue; }
+					h1 { color: var(--foo); background: var(--baz); }
+					h2 { color: var(--baz); background: var(--foo); }`,
+			},
+		];
+		const { results } = audit( files );
+		expect( getResultValue( results, 'unused' ) ).toHaveLength( 1 );
+		expect( getResultValue( results, 'undefined' ) ).toHaveLength( 1 );
+	} );
+
+	it( 'should count custom properties used in other custom properties', () => {
+		const files = [
+			{
+				name: 'a.css',
+				content: `:root { --foo: red; --bar: 1px solid var(--foo); }
+					h1 { border: var(--bar); }`,
+			},
+		];
+		const { results } = audit( files );
+		expect( getResultValue( results, 'unused' ) ).toHaveLength( 0 );
+		expect( getResultValue( results, 'undefined' ) ).toHaveLength( 0 );
+	} );
+
+	it( 'should count the number of custom properties across multiple files', () => {
+		const files = [
+			{
+				name: 'props.css',
+				content: `:root { --foo: red; --bar: blue; }`,
+			},
+			{
+				name: 'a.css',
+				content: `h1 { color: var(--foo); }`,
+			},
+			{
+				name: 'b.css',
+				content: `h2 { color: rgb(var(--baz) / 0.5); background: var(--foo); }`,
+			},
+		];
+		const { results } = audit( files );
+		expect( getResultValue( results, 'unused' ) ).toHaveLength( 1 );
+		expect( getResultValue( results, 'undefined' ) ).toHaveLength( 1 );
+	} );
+} );
diff --git a/src/audits/custom-properties.js b/src/audits/custom-properties.js
new file mode 100644
index 0000000..c78e881
--- /dev/null
+++ b/src/audits/custom-properties.js
@@ -0,0 +1,55 @@
+/**
+ * External dependencies
+ */
+const { parse } = require( 'postcss' );
+const { parse: parseValue } = require( 'postcss-values-parser' );
+
+module.exports = function ( files = [] ) {
+	const definedProps = [];
+	const usedProps = [];
+	files.forEach( ( { content, name } ) => {
+		const root = parse( content, { from: name } );
+		root.walkDecls( ( { prop, value } ) => {
+			if ( prop ) {
+				if ( '--' === prop.slice( 0, 2 ) ) {
+					if ( ! definedProps.includes( prop ) ) {
+						definedProps.push( prop );
+					}
+				}
+				try {
+					const valueRoot = parseValue( value, {
+						ignoreUnknownWords: true,
+					} );
+					valueRoot.walkFuncs( ( node ) => {
+						if ( node.isVar ) {
+							if ( ! usedProps.includes( node.first.value ) ) {
+								usedProps.push( node.first.value );
+							}
+						}
+					} );
+				} catch ( error ) {}
+			}
+		} );
+	} );
+
+	return {
+		audit: 'custom-props',
+		name: 'Custom Properties',
+		results: [
+			{
+				id: 'unused',
+				label: 'Defined but unused custom properties',
+				value: definedProps
+					.filter( ( x ) => ! usedProps.includes( x ) )
+					.map( ( prop ) => ( { name: prop } ) ),
+			},
+			{
+				id: 'undefined',
+				label: 'Undefined custom properties',
+				value: usedProps
+					.filter( ( x ) => ! definedProps.includes( x ) )
+					.map( ( prop ) => ( { name: prop } ) ),
+			},
+		],
+	};
+};
diff --git a/src/formats/html/_audit-default.twig b/src/formats/html/_audit-default.twig
index b7acced..80a670d 100644
--- a/src/formats/html/_audit-default.twig
+++ b/src/formats/html/_audit-default.twig
@@ -5,7 +5,9 @@
 		{% for value in item.value %}
 			{% if value.name %}
 				<li>
-					<span class="count">{{value.count}}</span>
+					{% if value.count %}
+						<span class="count">{{value.count}}</span>
+					{% endif %}
 					<em>{{value.name}}</em>
 				</li>
 			{% endif %}
@@ -21,4 +23,4 @@
 	{% else %}
 		<h3>{{item.label}}: <span class="count">{{item.value}}</span></h3>
 	{% endif %}
-{% endfor %}
\ No newline at end of file
+{% endfor %}
diff --git a/src/run.js b/src/run.js
index 0856c73..e2812cd 100644
--- a/src/run.js
+++ b/src/run.js
@@ -27,6 +27,9 @@ const runAudits = ( cssFiles ) => {
 	if ( getArg( '--typography' ) ) {
 		audits.push( require( './audits/typography' )( cssFiles ) );
 	}
+	if ( getArg( '--custom-props' ) ) {
+		audits.push( require( './audits/custom-properties' )( cssFiles ) );
+	}
 
 	const propertyValues = getArg( '--property-values' );
 	const isPropertyValuesArray =