This book deals with neutrino physics and astrophysics - a field in which some of the most exciting recent developments in particle physics, astrophysics and cosmology took place. The book is the most up-to-date, comprehensive and self-contained treatment of key issues in neutrino physics. It discusses all the topics vital to the understanding of the nature of neutrinos such as what they are, how to describe them, how they behave in nature, and the roles that neutrinos play in shaping our Universe. The book provides comprehensive discussions, both experimental and theoretical, with relevant mathematical details, on neutrino oscillations, extra-terrestrial as well as terrestrial neutrinos and the relic neutrinos. It also discusses many implications of current experimental data on reactor, accelerator, atmospheric, solar and supernova neutrinos with future perspectives. The book starts with an introduction to field theory and gauge theory which is accessible even to advanced undergraduate students, with helpful appendices, and it also provides pedagogical but sufficiently detailed reviews of supernova physics and cosmology, in particular the Cosmic Microwave Background Radiation. It aims to provide all the technical details necessary for the professionals in the field and to be an almost exhaustive reference for neutrino physicists with over 1000 references.