diff --git a/src/rules/define-tag-after-class-definition.ts b/src/rules/define-tag-after-class-definition.ts
new file mode 100644
index 0000000..c7275e2
--- /dev/null
+++ b/src/rules/define-tag-after-class-definition.ts
@@ -0,0 +1,89 @@
+/**
+ * @fileoverview Enforces that the `define(...)` call happens after the
+ * associated class has been defined
+ * @author James Garbutt <https://github.com/43081j>
+ */
+
+import {Rule} from 'eslint';
+import * as ESTree from 'estree';
+import {isCustomElement, isDefineCall} from '../util';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+const rule: Rule.RuleModule = {
+  meta: {
+    docs: {
+      description:
+        'Enforces that the `define(...)` call happens after' +
+        ' the associated class has been defined',
+      url: 'https://github.com/43081j/eslint-plugin-wc/blob/master/docs/rules/define-tag-after-class-definition.md'
+    },
+    messages: {
+      unregistered:
+        'Custom element class has not been registered with ' +
+        ' a `customElements.define` call',
+      noExpressions:
+        'Custom element classes should not be declared inline. ' +
+        'They should be exported as concrete class declarations.'
+    }
+  },
+
+  create(context): Rule.RuleListener {
+    const seenClasses = new Set<ESTree.Node>();
+    const source = context.getSourceCode();
+
+    //----------------------------------------------------------------------
+    // Helpers
+    //----------------------------------------------------------------------
+
+    //----------------------------------------------------------------------
+    // Public
+    //----------------------------------------------------------------------
+
+    return {
+      'ClassDeclaration,ClassExpression': (node: ESTree.Class): void => {
+        if (
+          isCustomElement(context, node, source.getJSDocComment(node)) &&
+          node.id?.type === 'Identifier'
+        ) {
+          seenClasses.add(node);
+        }
+      },
+      CallExpression: (node: ESTree.CallExpression): void => {
+        const tagClass = node.arguments[1];
+
+        if (isDefineCall(node)) {
+          if (tagClass.type === 'Identifier') {
+            const ref = context
+              .getScope()
+              .references.find((r) => r.identifier.name === tagClass.name);
+
+            if (ref?.resolved && ref.resolved.defs.length === 1) {
+              seenClasses.delete(ref.resolved.defs[0].node);
+            }
+          } else if (tagClass.type === 'ClassExpression') {
+            seenClasses.delete(tagClass);
+            context.report({
+              node: tagClass,
+              messageId: 'noExpressions'
+            });
+          }
+        }
+      },
+      'Program:exit': (): void => {
+        for (const node of seenClasses) {
+          context.report({
+            node,
+            messageId: 'unregistered'
+          });
+        }
+
+        seenClasses.clear();
+      }
+    };
+  }
+};
+
+export default rule;
diff --git a/src/rules/no-invalid-element-name.ts b/src/rules/no-invalid-element-name.ts
index 1214b04..e28e980 100644
--- a/src/rules/no-invalid-element-name.ts
+++ b/src/rules/no-invalid-element-name.ts
@@ -7,6 +7,7 @@ import {Rule} from 'eslint';
 import * as ESTree from 'estree';
 import isValidElementName = require('is-valid-element-name');
 import {knownNamespaces} from '../util/tag-names';
+import {isDefineCall} from '../util';
 
 //------------------------------------------------------------------------------
 // Rule Definition
@@ -82,18 +83,7 @@ const rule: Rule.RuleModule = {
 
     return {
       CallExpression: (node: ESTree.CallExpression): void => {
-        if (
-          node.callee.type === 'MemberExpression' &&
-          ((node.callee.object.type === 'MemberExpression' &&
-            node.callee.object.object.type === 'Identifier' &&
-            node.callee.object.object.name === 'window' &&
-            node.callee.object.property.type === 'Identifier' &&
-            node.callee.object.property.name === 'customElements') ||
-            (node.callee.object.type === 'Identifier' &&
-              node.callee.object.name === 'customElements')) &&
-          node.callee.property.type === 'Identifier' &&
-          node.callee.property.name === 'define'
-        ) {
+        if (isDefineCall(node)) {
           const firstArg = node.arguments[0];
           if (
             firstArg &&
diff --git a/src/test/rules/define-tag-after-class-definition_test.ts b/src/test/rules/define-tag-after-class-definition_test.ts
new file mode 100644
index 0000000..899d8f0
--- /dev/null
+++ b/src/test/rules/define-tag-after-class-definition_test.ts
@@ -0,0 +1,134 @@
+/**
+ * @fileoverview Enforces that the `define(...)` call happens after the
+ * associated class has been defined
+ * @author James Garbutt <https://github.com/43081j>
+ */
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+import rule from '../../rules/define-tag-after-class-definition';
+import {RuleTester} from 'eslint';
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    sourceType: 'module',
+    ecmaVersion: 2015
+  }
+});
+
+ruleTester.run('define-tag-after-class-definition', rule, {
+  valid: [
+    'class Foo {}',
+    `class Foo extends HTMLElement {}
+    customElements.define('x-foo', Foo);`,
+    `class Foo extends HTMLElement {}
+    const x = 303;
+    customElements.define('x-foo', Foo);`,
+    `class Foo extends HTMLElement {}
+    window.customElements.define('x-foo', Foo);`,
+    `/** @customElement x-foo */
+    class Foo extends Bar {}
+    customElements.define('x-foo', Foo);`,
+    `customElements.define('x-foo', Foo);`,
+    `const Foo = doSomeBlackBoxThing();
+    customElements.define('x-foo', Foo);`,
+    {
+      code: `class Foo extends Bar {}
+      customElements.define('x-foo', Foo);`,
+      settings: {
+        wc: {
+          elementBaseClasses: ['Bar']
+        }
+      }
+    }
+  ],
+
+  invalid: [
+    {
+      code: 'class Foo extends HTMLElement {}',
+      errors: [
+        {
+          messageId: 'unregistered',
+          line: 1,
+          column: 1
+        }
+      ]
+    },
+    {
+      code: `class Foo extends HTMLElement {}
+      customElements.define('x-foo', someThingElse);`,
+      errors: [
+        {
+          messageId: 'unregistered',
+          line: 1,
+          column: 1
+        }
+      ]
+    },
+    {
+      code: `class Foo extends HTMLElement {}
+      customElements.define('x-foo', someDynamicThing());`,
+      errors: [
+        {
+          messageId: 'unregistered',
+          line: 1,
+          column: 1
+        }
+      ]
+    },
+    {
+      code: `class Foo extends HTMLElement {}
+        someOtherNonsense.define('x-foo', Foo);`,
+      errors: [
+        {
+          messageId: 'unregistered',
+          line: 1,
+          column: 1
+        }
+      ]
+    },
+    {
+      code: `customElements.define('x-foo', class extends HTMLElement {
+      });`,
+      errors: [
+        {
+          messageId: 'noExpressions',
+          line: 1,
+          column: 32
+        }
+      ]
+    },
+    {
+      code: 'class Foo extends Bar {}',
+      settings: {
+        wc: {
+          elementBaseClasses: ['Bar']
+        }
+      },
+      errors: [
+        {
+          messageId: 'unregistered',
+          line: 1,
+          column: 1
+        }
+      ]
+    },
+    {
+      code: `/** @customElement x-foo */
+      class Foo extends Bar {}`,
+      errors: [
+        {
+          messageId: 'unregistered',
+          line: 2,
+          column: 7
+        }
+      ]
+    }
+  ]
+});
diff --git a/src/util.ts b/src/util.ts
index d7d4203..196154a 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -89,3 +89,23 @@ export function isNativeCustomElement(node: ESTree.Class): boolean {
     node.superClass.name === 'HTMLElement'
   );
 }
+
+/**
+ * Determines if a call expression is a `customElements.define` call
+ * @param {ESTree.CallExpression} node Node to test
+ * @return {boolean}
+ */
+export function isDefineCall(node: ESTree.CallExpression): boolean {
+  return (
+    node.callee.type === 'MemberExpression' &&
+    ((node.callee.object.type === 'MemberExpression' &&
+      node.callee.object.object.type === 'Identifier' &&
+      node.callee.object.object.name === 'window' &&
+      node.callee.object.property.type === 'Identifier' &&
+      node.callee.object.property.name === 'customElements') ||
+      (node.callee.object.type === 'Identifier' &&
+        node.callee.object.name === 'customElements')) &&
+    node.callee.property.type === 'Identifier' &&
+    node.callee.property.name === 'define'
+  );
+}