In this chapter, the advantages of using ammonia as the nitrogen source for molecular beam epitaxy (MBE) growth are discussed. Then, a description of the growth of AlGaN/GaN HEMT heterostructures on GaN-on-sapphire templates, free-standing GaN and on foreign substrates like silicon and silicon carbide is provided. The behavior of transistor devices is described and high-frequency power density results are presented. For transistor applications on silicon, the growth requires even more attention due to possible conductivity through the substrate. In this context, the benefit of reducing the growth temperature of III-nitrides can be seen. Finally, a technological route for the monolithic integration of GaN with silicon electron devices in a complementary metal-oxide-semiconductor (CMOS)-first approach is described.