One of the most important physics question is, whether neutrinos are Majorana particles, i.e., they are their own antiparticles. This will also mean that there is lepton number violation in nature. This will have several interesting consequences in astrophysics and cosmology.

Search for the Neutrinoless double beta decay can answer this question and provide the absolute mass of the neutrinos, which is the most important unknown parameter in neutrino physics at present. In this effort the main problem is to detect this extremely rare process. In addition, proper interpretation of the results require calculation of complicated nuclear structure matrix elements from nuclear models. Considering the importance, many new neutrinoless double beta decay experiments are planned all over the world. In India also there is an effort to perform this experiment in the underground laboratory of the proposed India-based Neutrino Observatory. In this book this and other related experiments are reviewed and also the theoretical aspects of the neutrinoless double beta decay and its consequences are summarized.