diff --git a/packages/@stimulus/core/src/application.ts b/packages/@stimulus/core/src/application.ts
index dc17ee0f..df52be88 100644
--- a/packages/@stimulus/core/src/application.ts
+++ b/packages/@stimulus/core/src/application.ts
@@ -13,6 +13,7 @@ export class Application implements ErrorHandler {
   readonly router: Router
   logger: Logger = console
   debug: boolean = false
+  warnings: boolean = true
 
   static start(element?: Element, schema?: Schema): Application {
     const application = new Application(element, schema)
@@ -71,6 +72,14 @@ export class Application implements ErrorHandler {
     return context ? context.controller : null
   }
 
+  // Warning handling
+
+  handleWarning(warning: string, message: string, detail: object) {
+    if (this.warnings) {
+      this.logger.warn(`%s\n\n%s\n\n%o`, message, warning, detail)
+    }
+  }
+
   // Error handling
 
   handleError(error: Error, message: string, detail: object) {
diff --git a/packages/@stimulus/core/src/binding_observer.ts b/packages/@stimulus/core/src/binding_observer.ts
index 1456cd21..6356054c 100644
--- a/packages/@stimulus/core/src/binding_observer.ts
+++ b/packages/@stimulus/core/src/binding_observer.ts
@@ -59,8 +59,18 @@ export class BindingObserver implements ValueListObserverDelegate<Action> {
 
   private connectAction(action: Action) {
     const binding = new Binding(this.context, action)
+    const { controller } = binding.context
+    const method = (controller as any)[action.methodName]
+
     this.bindingsByAction.set(action, binding)
     this.delegate.bindingConnected(binding)
+
+    if (typeof method != "function") {
+      this.context.handleWarning(
+        `Action "${action.toString()}" references undefined method "${action.methodName}" on controller "${action.identifier}"`,
+        `connecting action "${action.toString()}"`
+      )
+    }
   }
 
   private disconnectAction(action: Action) {
@@ -92,4 +102,12 @@ export class BindingObserver implements ValueListObserverDelegate<Action> {
   elementUnmatchedValue(element: Element, action: Action) {
     this.disconnectAction(action)
   }
+
+  elementMatchedNoValue(token: Token) {
+    const action = Action.forToken(token)
+    this.context.handleWarning(
+      `Action "${token.content}" references undefined controller "${action.identifier}"`,
+      `connecting action "${token.content}"`
+    )
+  }
 }
diff --git a/packages/@stimulus/core/src/context.ts b/packages/@stimulus/core/src/context.ts
index 49b8f6a6..2605fa4d 100644
--- a/packages/@stimulus/core/src/context.ts
+++ b/packages/@stimulus/core/src/context.ts
@@ -6,12 +6,14 @@ import { ErrorHandler } from "./error_handler"
 import { Module } from "./module"
 import { Schema } from "./schema"
 import { Scope } from "./scope"
+import { TargetGuide } from "./target_guide"
 import { ValueObserver } from "./value_observer"
 
 export class Context implements ErrorHandler {
   readonly module: Module
   readonly scope: Scope
   readonly controller: Controller
+  readonly targetGuide: TargetGuide
   private bindingObserver: BindingObserver
   private valueObserver: ValueObserver
 
@@ -19,6 +21,7 @@ export class Context implements ErrorHandler {
     this.module = module
     this.scope = scope
     this.controller = new module.controllerConstructor(this)
+    this.targetGuide = new TargetGuide(this.scope, this.controller)
     this.bindingObserver = new BindingObserver(this, this.dispatcher)
     this.valueObserver = new ValueObserver(this, this.controller)
 
@@ -86,7 +89,13 @@ export class Context implements ErrorHandler {
     this.application.handleError(error, `Error ${message}`, detail)
   }
 
-  // Debug logging
+  // Logging
+
+  handleWarning(warning: string, message: string, detail: object = {}) {
+    const { identifier, controller, element } = this
+    detail = Object.assign({ identifier, controller, element }, detail)
+    this.application.handleWarning(warning, `Warning ${message}`, detail)
+  }
 
   logDebugActivity = (functionName: string, detail: object = {}): void => {
     const { identifier, controller, element } = this
diff --git a/packages/@stimulus/core/src/router.ts b/packages/@stimulus/core/src/router.ts
index 8db0c92b..60cda1d3 100644
--- a/packages/@stimulus/core/src/router.ts
+++ b/packages/@stimulus/core/src/router.ts
@@ -59,6 +59,7 @@ export class Router implements ScopeObserverDelegate {
 
   unloadIdentifier(identifier: string) {
     const module = this.modulesByIdentifier.get(identifier)
+
     if (module) {
       this.disconnectModule(module)
     }
@@ -84,10 +85,17 @@ export class Router implements ScopeObserverDelegate {
   }
 
   scopeConnected(scope: Scope) {
-    this.scopesByIdentifier.add(scope.identifier, scope)
-    const module = this.modulesByIdentifier.get(scope.identifier)
+    const { identifier, element } = scope
+    this.scopesByIdentifier.add(identifier, scope)
+    const module = this.modulesByIdentifier.get(identifier)
     if (module) {
       module.connectContextForScope(scope)
+    } else {
+      this.application.handleWarning(
+        `Element references undefined controller "${identifier}"`,
+        `Warning connecting controller "${identifier}"`,
+        { identifier, element }
+      )
     }
   }
 
diff --git a/packages/@stimulus/core/src/scope_observer.ts b/packages/@stimulus/core/src/scope_observer.ts
index 85f9746f..541b4f0c 100644
--- a/packages/@stimulus/core/src/scope_observer.ts
+++ b/packages/@stimulus/core/src/scope_observer.ts
@@ -71,6 +71,8 @@ export class ScopeObserver implements ValueListObserverDelegate<Scope> {
     }
   }
 
+  elementMatchedNoValue(token: Token) {}
+
   private fetchScopesByIdentifierForElement(element: Element) {
     let scopesByIdentifier = this.scopesByIdentifierByElement.get(element)
     if (!scopesByIdentifier) {
diff --git a/packages/@stimulus/core/src/target_guide.ts b/packages/@stimulus/core/src/target_guide.ts
new file mode 100644
index 00000000..12692644
--- /dev/null
+++ b/packages/@stimulus/core/src/target_guide.ts
@@ -0,0 +1,123 @@
+import { Controller } from "./controller"
+import { Constructor } from "./constructor"
+import { Scope } from "./scope"
+import { TargetDescriptor } from "./target_properties"
+import { readInheritableStaticArrayValues } from "./inheritable_statics"
+
+export class TargetGuide {
+  readonly scope: Scope
+  readonly controller: Controller
+  readonly definedTargets: Array<String>
+
+  constructor(scope: Scope, controller: Controller) {
+    this.scope = scope
+    this.controller = controller
+    this.definedTargets = readInheritableStaticArrayValues(this.controller.constructor as Constructor<Controller>, "targets")
+
+    this.searchForUndefinedTargets()
+  }
+
+  get identifier() {
+    return this.scope.identifier
+  }
+
+  get targets() {
+    return this.scope.targets
+  }
+
+  get registeredControllers(): Array<String> {
+    return this.controller.application.router.modules.map((c) => c.identifier)
+  }
+
+  controllerRegistered(controllerName: string): Boolean {
+    return this.registeredControllers.includes(controllerName)
+  }
+
+  targetDefined(targetName: string): Boolean {
+    return this.definedTargets.includes(targetName)
+  }
+
+  private getAllTargets(element: Element): Array<TargetDescriptor> {
+    const attribute = `data-${this.identifier}-target`
+    const selector = `[${attribute}]`
+
+    return Array.from(element.querySelectorAll(selector)).map((element: Element) => {
+      const target = element.getAttribute(attribute)
+      return {
+        identifier: this.identifier,
+        target,
+        element,
+        attribute,
+        legacy: false
+      }
+    })
+  }
+
+  private getAllLegacyTargets(element: Element): Array<TargetDescriptor> {
+    const attribute = "data-target"
+    const selector = `[${attribute}]`
+
+    return Array.from(element.querySelectorAll(selector)).map((element: Element) => {
+      const value = element.getAttribute(attribute)
+      const parts = value ? value.split(".") : []
+      return {
+        identifier: parts[0],
+        target: parts[1],
+        element,
+        attribute,
+        legacy: true
+      }
+    })
+  }
+
+  private searchForUndefinedTargets() {
+    const { element } = this.scope
+
+    const targets: Array<TargetDescriptor> = [
+      ...this.getAllTargets(element),
+      ...this.getAllLegacyTargets(element)
+    ]
+
+    targets.forEach((descriptor) => {
+      const { identifier, attribute, target, legacy, element } = descriptor
+
+      if (identifier && target) {
+        this.handleWarningForUndefinedTarget(descriptor)
+      } else if (identifier && !target) {
+        this.controller.context.handleWarning(
+          `The "${attribute}" attribute of the Element doesn't include a target. Please specify a target for the "${identifier}" controller.`,
+          `connecting target for "${identifier}"`,
+          { identifier, target, attribute, element }
+        )
+      } else if (legacy && !target) {
+        this.controller.context.handleWarning(
+          `The "${attribute}" attribute of the Element doesn't include a value. Please specify a controller and target value in the right format.`,
+          `connecting target`,
+          { identifier, target, attribute, element }
+        )
+      }
+    })
+  }
+
+  private handleWarningForUndefinedTarget(descriptor: TargetDescriptor) {
+    const { identifier, target, element, attribute } = descriptor
+
+    if (identifier === this.identifier) {
+      if (!this.targetDefined(target as string)) {
+        this.controller.context.handleWarning(
+          `Element references undefined target "${target}" for controller "${identifier}". Make sure you defined the target "${target}" in the "static targets" array of your controller.`,
+          `connecting target "${identifier}.${target}"`,
+          { identifier, target, element, attribute }
+        )
+      }
+    } else {
+      if (!this.controllerRegistered(identifier as string)) {
+        this.controller.context.handleWarning(
+          `Target "${target}" references undefined controller "${identifier}". Make sure you registered the "${identifier}" controller.`,
+          `connecting target "${identifier}.${target}"`,
+          { identifier, target, element, attribute }
+        )
+      }
+    }
+  }
+}
diff --git a/packages/@stimulus/core/src/target_properties.ts b/packages/@stimulus/core/src/target_properties.ts
index 8aad1af6..e02fd921 100644
--- a/packages/@stimulus/core/src/target_properties.ts
+++ b/packages/@stimulus/core/src/target_properties.ts
@@ -10,6 +10,14 @@ export function TargetPropertiesBlessing<T>(constructor: Constructor<T>) {
   }, {} as PropertyDescriptorMap)
 }
 
+export type TargetDescriptor = {
+  identifier: string | null
+  target: string | null
+  attribute: string
+  legacy: Boolean
+  element: Element
+}
+
 function propertiesForTargetDefinition(name: string) {
   return {
     [`${name}Target`]: {
diff --git a/packages/@stimulus/core/src/tests/cases/application_test_case.ts b/packages/@stimulus/core/src/tests/cases/application_test_case.ts
index cc516cf0..0649d925 100644
--- a/packages/@stimulus/core/src/tests/cases/application_test_case.ts
+++ b/packages/@stimulus/core/src/tests/cases/application_test_case.ts
@@ -16,6 +16,7 @@ export class ApplicationTestCase extends DOMTestCase {
     try {
       this.setupApplication()
       this.application.start()
+      this.application.warnings = false
       await super.runTest(testName)
     } finally {
       this.application.stop()
diff --git a/packages/@stimulus/mutation-observers/src/tests/cases/value_list_observer_tests.ts b/packages/@stimulus/mutation-observers/src/tests/cases/value_list_observer_tests.ts
index 242dfb7c..7b1852c3 100644
--- a/packages/@stimulus/mutation-observers/src/tests/cases/value_list_observer_tests.ts
+++ b/packages/@stimulus/mutation-observers/src/tests/cases/value_list_observer_tests.ts
@@ -103,6 +103,10 @@ export default class ValueListObserverTests extends ObserverTestCase implements
     this.recordCall("elementMatchedValue", element, value.id, value.token.content)
   }
 
+  elementMatchedNoValue(token: Token) {
+    this.recordCall("elementMatchedNoValue", token)
+  }
+
   elementUnmatchedValue(element: Element, value: Value) {
     this.recordCall("elementUnmatchedValue", element, value.id, value.token.content)
   }
diff --git a/packages/@stimulus/mutation-observers/src/value_list_observer.ts b/packages/@stimulus/mutation-observers/src/value_list_observer.ts
index f1cf28be..2ba2fb94 100644
--- a/packages/@stimulus/mutation-observers/src/value_list_observer.ts
+++ b/packages/@stimulus/mutation-observers/src/value_list_observer.ts
@@ -4,6 +4,7 @@ export interface ValueListObserverDelegate<T> {
   parseValueForToken(token: Token): T | undefined
   elementMatchedValue(element: Element, value: T): void
   elementUnmatchedValue(element: Element, value: T): void
+  elementMatchedNoValue(token: Token): void
 }
 
 interface ParseResult<T> {
@@ -54,6 +55,8 @@ export class ValueListObserver<T> implements TokenListObserverDelegate {
     if (value) {
       this.fetchValuesByTokenForElement(element).set(token, value)
       this.delegate.elementMatchedValue(element, value)
+    } else {
+      this.delegate.elementMatchedNoValue(token)
     }
   }
 
diff --git a/packages/@stimulus/polyfills/index.js b/packages/@stimulus/polyfills/index.js
index 5601543d..30f9808a 100644
--- a/packages/@stimulus/polyfills/index.js
+++ b/packages/@stimulus/polyfills/index.js
@@ -1,6 +1,7 @@
 import "core-js/es/array/find"
 import "core-js/es/array/find-index"
 import "core-js/es/array/from"
+import "core-js/es/array/includes"
 import "core-js/es/map"
 import "core-js/es/object/assign"
 import "core-js/es/promise"