What are Core and Shared modules, and when do you use them?

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Two conventional Angular module types for organizing larger apps. The standalone migration is making them less common, but still good interview knowledge. CoreModule — singletons that load once at app startup: auth service, HTTP interceptors, header/footer, error handlers. Imported only by AppModule . SharedModule — reusable components, directives, pipes used in many feature modules: buttons, modals, formatting pipes. Imported by every feature module that needs them. Why split — clear ownership: Core = app-wide singletons; Shared = reusable presentational pieces. Don't put services in Shared — each importer gets its own instance; defeats singleton intent. Modern alternative — standalone components + providedIn: "root" services; Core/Shared boilerplate fades. Core = singletons imported once (auth, interceptors); Shared = reusables imported everywhere (buttons, pipes). `@SkipSelf()` import guard blocks double-instantiation. Less relevant with `providedIn: 'root'`.