For many years neutrinos were considered as massless particles. The theory of a two-component neutrino, which played a crucial role in the weak interaction, is based on the assumption that neutrinos are massless. We now know that neutrinos have nonzero small masses and they play an important role in many areas of physics. These particles that complement the efforts of laboratory experiments are now an active research field providing information about the hidden shape of nature. The subject of this book focuses on the physics of neutrinos. An overview of their histroy and properties are given, their recent established status within the standard model of particle physics is presented and their phenomenology probing new physics are discussed.