This volume presents the progress made in the study of the Seesaw mechanism and of its implications over the past 25 years, as well as the recent theoretical developments in the understanding of the measured pattern of neutrino masses and mixings. Observable implications of the Seesaw mechanism like leptogenesis or lepton flavour violation in supersymmetric theories are discussed, and the key aspects of the vast experimental program aimed at determining the neutrino parameters and properties are reviewed. The book provides an up-to-date and in-depth perspective on neutrino masses and on the associated physics in the Seesaw scenario.