diff --git a/packages/core/src/index.spec.ts b/packages/core/src/index.spec.ts
index 81ce71e..9397480 100644
--- a/packages/core/src/index.spec.ts
+++ b/packages/core/src/index.spec.ts
@@ -26,6 +26,7 @@ class UserProvider extends BaseService {
 describe('Instantiation', () => {
   it('#getSingleton should instantiate a singleton class once', () => {
     let app = new App();
+    app.provideSingleton(Database);
     let db1 = app.getSingleton(Database);
     let db2 = app.getSingleton(Database);
     expect(db1.id).toEqual(db2.id);
@@ -33,7 +34,9 @@ describe('Instantiation', () => {
 
   it('#getSingleton should instantiate a singleton factory once', () => {
     let app = new App();
-    let factoryFunc = () => app.getSingleton(Database);
+    let i = 0;
+    let factoryFunc = (_app: App) => ({ id: ++i });
+    app.provideSingleton(factoryFunc);
     let db1 = app.getSingleton(factoryFunc);
     let db2 = app.getSingleton(factoryFunc);
     expect(db1.id).toEqual(db2.id);
@@ -42,14 +45,13 @@ describe('Instantiation', () => {
   it('#load should force a singleton to instantitate', () => {
     let app = new App();
     let dbDidInit: boolean = false;
-    app.load(
-      class MyDB extends Database {
-        constructor(app: App) {
-          super(app);
-          dbDidInit = true;
-        }
-      },
-    );
+    class MyDB extends Database {
+      constructor(app: App) {
+        super(app);
+        dbDidInit = true;
+      }
+    }
+    app.load(MyDB);
     expect(dbDidInit).toBe(true);
   });
 });
@@ -57,6 +59,7 @@ describe('Instantiation', () => {
 describe('Overrides', () => {
   it('#getSingleton should use and respect singleton overrides', () => {
     let app = new App();
+    app.provideSingleton(Database);
     app.overrideSingleton(
       Database,
       class MockDb extends Database {
@@ -98,6 +101,7 @@ describe('Overrides', () => {
 
   it('#clearSingletonOverrides should cause original singletons to instantiate', () => {
     let app = new App();
+    app.provideSingleton(Database);
     app.overrideSingleton(
       Database,
       class MockDb extends Database {
@@ -112,6 +116,7 @@ describe('Overrides', () => {
 describe('App nesting', () => {
   it('#getSingleton should find instantiated singletons in a parent app', () => {
     let app = new App();
+    app.provideSingleton(Database);
     let dbId = app.getSingleton(Database).id;
     let childApp = app.createChildApp();
     expect(childApp.getSingleton(Database).id).toEqual(dbId);
@@ -119,6 +124,7 @@ describe('App nesting', () => {
 
   it('#getSingleton should instantiate non-existing singletons in the child app, not parent', () => {
     let parentApp = new App();
+    parentApp.provideSingleton(Database);
     let childApp = parentApp.createChildApp();
     let childDbId = childApp.getSingleton(Database).id;
     expect(parentApp.getSingleton(Database).id !== childDbId);
@@ -174,3 +180,60 @@ describe('Context disposal', () => {
       );
   });
 });
+
+describe('providing dependencies', () => {
+  it('should throw when getting singletons that arent provided', () => {
+    let app = new App();
+    class MySingleton extends AppSingleton {}
+    expect(() => app.getSingleton(MySingleton)).toThrowError();
+  });
+
+  it('should not throw when getting provided singletons', () => {
+    let app = new App();
+    class MySingleton extends AppSingleton {}
+    let mySingletonModule = (app: App) => app.provideSingleton(MySingleton);
+    app.load(mySingletonModule);
+    app.getSingleton(MySingleton); // shouldn't throw
+  });
+
+  it('should throw when you provide singletons twice', () => {
+    let app = new App();
+    class MySingleton extends AppSingleton {}
+    app.provideSingleton(MySingleton);
+    expect(() => app.provideSingleton(MySingleton)).toThrow();
+  });
+
+  it('dependencies provided in child apps shouldnt affect parent apps', () => {
+    let app = new App();
+    let childApp = app.createChildApp();
+    class MySingleton extends AppSingleton {}
+    let myPlugin = (app: App) => {
+      app.provideSingleton(MySingleton);
+    };
+    childApp.load(myPlugin);
+    expect(() => app.getSingleton(MySingleton)).toThrow();
+  });
+
+  it('dependencies provided in the parent should be present in child apps', () => {
+    let app = new App();
+    class MySingleton extends AppSingleton {}
+    app.provideSingleton(MySingleton);
+    let childApp = app.createChildApp();
+    childApp.getSingleton(MySingleton); // should not throw
+  });
+
+  it('should throw when you require unprovided singletons', () => {
+    let app = new App();
+    class MySingleton extends AppSingleton {}
+    expect(() => app.requireSingleton(MySingleton)).toThrow();
+  });
+});
+
+describe('loading plugins', () => {
+  it('should throw when loading plugins twice', () => {
+    let app = new App();
+    let myPlugin = (app: App) => app.provideSingleton(class MySingleton extends AppSingleton {});
+    app.load(myPlugin);
+    expect(() => app.load(myPlugin)).toThrow();
+  });
+});
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index e86183f..24d09a5 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -46,11 +46,18 @@ export type PublicInterface<T> = { [K in keyof T]: T[K] };
  */
 export class App {
   private singletonLocator: Locator<this>;
+  private providedSingletons: WeakMap<ConstructorOrFactory<App, any>, boolean> = new WeakMap();
   parentApp: App | null;
 
   constructor(opts: { parentApp?: App } = {}) {
     this.parentApp = opts.parentApp ? opts.parentApp : null;
     this.singletonLocator = new Locator(this, s => '__appSingleton' in s);
+
+    if (!opts.parentApp) {
+      // we must provide this, otherwise withServiceContext will fail every time.
+      // we do it once, on the parent app, because child app construction will fail otherwise.
+      this.provideSingleton(ServiceContextEvents); // otherwise, withServiceContext fails.
+    }
   }
 
   /**
@@ -102,6 +109,42 @@ export class App {
     }
   }
 
+  /**
+   * Registers a singleton as "provided". It's informing the application that a plugin
+   * agreed to expose that singleton.
+   *
+   * Calls to app.getSingleton(UnprovidedService) will fail with an error.
+   *
+   * Also see: `App#requireSingleton`
+   */
+  provideSingleton<T>(Klass: ConstructorOrFactory<App, T>): void {
+    if (this.isSingletonProvided(Klass)) {
+      throw new Error(`The singleton ${Klass.name} is already provided.`);
+    }
+    this.providedSingletons.set(Klass, true);
+  }
+
+  private isSingletonProvided<T>(Klass: ConstructorOrFactory<App, T>): boolean {
+    if (this.providedSingletons.has(Klass)) {
+      return true;
+    } else if (this.parentApp) {
+      return this.parentApp.isSingletonProvided(Klass);
+    } else {
+      return false;
+    }
+  }
+
+  /**
+   * Ensures that the singleton is provided; throws if it's not.
+   *
+   * Use this to detect unprovided but used singletons early.
+   */
+  requireSingleton<T>(Klass: ConstructorOrFactory<App, T>): void {
+    if (!this.isSingletonProvided(Klass)) {
+      throw new Error(`The singleton ${Klass} is required, but wasnt provided.`);
+    }
+  }
+
   /**
    * Returns an instance of the singleton, if it exists somewhere here or
    * in some of the parent apps. If it doesn't it's created in this app.
@@ -115,6 +158,12 @@ export class App {
     if (this.hasSingleton(Klass)) {
       return this.getExistingSingleton(Klass);
     }
+    if (!this.isSingletonProvided(Klass)) {
+      throw new Error(`Singleton ${Klass.name} wasnt provided`);
+      // console.warn(`The singleton ${Klass} was constructed, but wasn't provided beforehand.`);
+      // console.warn(`Please provide it explicitly using "App#provideSingleton(${Klass})".`);
+      // console.warn('In the future, this will be an error.');
+    }
     return this.singletonLocator.get(Klass);
   }
 
@@ -170,10 +219,16 @@ export class App {
    * While singleton classes are typically side effect free and can be instantiated lazily when
    * first requested, plugins have side-effects, such as adding router routes, adding RPC endpoints
    * or setting up event listeners. The load method is therefore used to load those plugins.
+   *
+   * You can load a plugin only once; load throws an error the second time.
    */
   load<T>(Klass: ConstructorOrFactory<App, T>): void {
-    this.getSingleton(Klass); // force initialization;
-    return;
+    if (this.isSingletonProvided(Klass)) {
+      throw new Error(`Singleton ${Klass.name} is already initialized.`);
+    } else {
+      this.provideSingleton(Klass);
+      this.getSingleton(Klass); // force initialization
+    }
   }
 
   /**
@@ -298,6 +353,10 @@ export class ServiceContext {
   getSingleton<T>(SingletonClass: ConstructorOrFactory<App, T>): T {
     return this.app.getSingleton(SingletonClass);
   }
+
+  requireSingleton<T>(SingletonClass: ConstructorOrFactory<App, T>) {
+    return this.app.requireSingleton(SingletonClass);
+  }
 }
 
 type ContextListener = (serviceCtx: ServiceContext, error: Error | null) => PromiseLike<void>;