This volume is the result of extensive research into the foundations of quantum mechanics, and presents a new formulation of quantum theory which resolves many existing problems. The formalism is experimentally motivated and is shown to be consistent with recent foundational approaches to quantum mechanics. It is based on new extensions of the theory of group representations, which are developed and illustrated in the text. This new approach establishes connections with quantum logic, philosophy, and the history of science, allowing a broad range of applications not only in physics, but also in such areas as signal processing and neuroscience. Audience: This book will be of interest to researchers and students. It is recommended as a supplementary textbook for advanced courses in quantum mechanics. A background in quantum mechanics and complex analysis is assumed.