This book presents the first introduction and review of a new subfield of optics and plasma physics, dubbed relativistic nonlinear optics. It discusses the fundamental science of the interactions of the highest intensity light ever produced with matter, resulting in relativistic plasmas. Discussed are novel physical phenomena that are of interest to scientists engaged in fundamental physics, nonlinear optics and high-power lasers. Also reviewed are applications to a new generation of radiation sources with unique characteristics. Another application discussed is an advanced concept for laser-driven space propulsion. Unlike other books on related topics, this is a single-author text that begins by introducing the fundamental concepts, then presents the most important special topics, and lastly reviews several timely applications.