Filling a real need for a book that can be used both as a textbook as well as a contemporary reference, this monograph is unique in its focus on the basic theory of linear and nonlinear light-matter interactions in metamaterials. As such, it covers various modern approaches to their design and fabrication, as well as a variety of novel applications enabled by these unusual materials. Written with a broad audience in mind, ranging from graduate students specializing in physics, optics and electrical engineering, to scientists and engineers conducting active research in nanophotonics or developing novel applications of metamaterials.