After the successes of the Standard Model have been thoroughly discussed and appreciated in the previous chapters, open questions will now be examined in more detail. These relate on one hand to problems of cosmology, such as dark matter, dark energy, and the baryon asymmetry in the early universe, whose solutions are expected in particle physics; on the other hand, there are also misunderstood aspects within the Standard Model itself that demand an explanation. Concrete theoretical models that go beyond the Standard Model and could shed light on some of these questions, such as the Grand Unification of interactions and supersymmetry, will be presented. A summary and an outlook into the future conclude the chapter and the book.