This chapter provides an extensive discussion of Grand Unified Theories (GUTs) and related subjects. It begins with the SU(5) GUT, its fermion multiplets, and the resulting transitions between leptons and quarks, which enable in particular proton decay. In this context, we discuss the baryon asymmetry in the Universe, as well as possible topological defects dating back to the early Universe, according to the Kibble mechanism, such as domain walls, cosmic strings, or magnetic monopoles. That takes us to a review of Dirac and ‘t Hooft–Polaykov monopoles, Julia–Zee dyons, and the effects named after Callan–Rubakov and Witten. Next we discuss fermion masses in the framework of the GUTs with the gauge groups SU(5) and Spin(10). Then we consider small unified theories (without QCD) with a variety of gauge groups. Finally, we summarize the status and prospects of the GUT approach.