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 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 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, with helpful appendices, and it also provides pedagogical, but sufficiently detailed, reviews of supernova physics and cosmology, in particular the Cosmic Microwave Background Radiation.