Aimed at a general mathematical audience, this book provides a careful exposition of developments in the theory of spectral decomposition. Bringing the reader from the basics up to the level of current research in the area at the time of writing, Lange and Wang present an accessible account of the techniques used in the theory and applications of decomposable operators and related classes of operators. The book begins with a discussion of criteria for decomposable and related types of operators, and an analysis that relates and distinguishes among them. Perturbation theory of decomposable and other operators, applications to classical Hilberty space operators, quasisimilarity, and a new class of weakly decomposable operators are also discussed. The book closes with an exposition of some classical theories on invariant subspaces for subdecomposable and hyponormal operators, and a presentation of the parallel spectral theory of commuting systems.