The question on whether neutrinos are their own antiparticles, Majorana neutrinos, is crucial not only for neutrino physics, but also for particle physics and cosmology. Searches for neutrinoless double beta decay are crucial to answer the question. The physics potential of experiments looking for double beta decay is discussed, which is broader than just determining the neutrino nature. The potential of determining the Majorana nature in case there are additional interactions beyond the Standard Model is also discussed. Finally, it is investigated if one can experimentally distinguish whether the dark matter particle is its own antiparticle or not.