Book excerpt: Neutrinos are the central thread in the study of many aspects of particle physics and astrophysics. Neutrino interactions test the standard electroweak theory and its TeV scale extensions, and examine the structure of the nucleon and of the CKM matrix. Searches for neutrino mass and other intrinsic properties probe new physics at very short distance scales. The weak interactions of neutrinos imply for them a unique role in studying the early universe, the core of the Sun, type II supernovae, and active galactic nuclei, and suggest the possibility of small neutrino masses contributing to the missing matter in the Universe, especially on very large distance scales.