Neutrinos have remained enigmatic particles since their prediction by W. Pauli to the present day. Their properties and interactions in the Standard Model have been extensively discussed in previous chapters. In this chapter, the focus will be on neutrino masses. The first experimental indications of massive neutrinos with mv > 0 and neutrino mixing were provided by oscillation experiments. The theoretical aspects of these oscillations in vacuum and in matter are therefore examined in the first section. Subsequently, the oscillation experiments and their results are discussed in more detail. At the end of the chapter, the question of how Dirac and Majorana masses can be theoretically described and what possibilities exist to extend the Standard Model with massive neutrinos will be addressed.