Available for the first time in soft cover, this book is a classic on the foundations of quantum theory. It examines the subject from a point of view that goes back to Heisenberg and Dirac, and whose definitive mathematical formulation is due to von Neumann. This view, which is essentially geometric and relies on the concept of symmetry, leads most naturally to the fundamental questions that are the basis of all attempts to understand the world of atomic and subatomic particles. The mathematical treatment of symmetry in quantum theory is based on the theory of group representations, and this book includes a self-contained treatment of the parts of this theory that are most useful in quantum physics.