Ramsey theory has emerged as a cohesive subdiscipline of combinatorial analysis since about 1970. The present work comprises a self-contained exposition of the central results of Ramsey theory, describes recent developments in the field, and explores the influence of outside disciplines such as applications of topological dynamics and a combinatorial approach to undecidability results. New to the Second Edition is a complete treatment of Shelah's 1987 proof as well as the original proof of Van der Waerden's theorem.