Are neutrinos massless particles as predicted by the standard model of electroweak interactions? There are however many reasons to suspect that neutrinos may indeed be massive and understanding the neutrino mass may hold the key to our understanding of physics beyond the standard model.This book provides a comprehensive overview of our present understanding of neutrino mass in unified gauge theories, its phenomenological and cosmological implications. It is divided into four parts. The first part presents a model independent discussion of the properties of the neutrino and nature of the neutrino masses. Part 2 deals with neutrino masses in unified and grand unified theories. In Part 3, phenomenological implications of neutrino mass is discussed. This includes neutrino oscillations, neutrinoless double beta decay and electromagnetic properties of neutrinos. Part 4 considers the solar neutrino puzzle, SN1987A and cosmological constraints on the neutrino properties. The book is meant for beginning and advanced graduate students in particle physics as well as active researchers in the field. It contains an extensive list of references to the original literature.