This is the first book devoted to quantum state diffusion (QSD) and its applications to open quantum systems and to the foundations of quantum mechanics. Recent experiments with detailed control over individual quantum systems have changed the face of quantum physics. These systems include atoms at the low temperatures attained by the 1997 Nobel Laureates, they include entangled photons in cavities, and they include the quantum systems used in new and future technologies like quantum cryptography and quantum computation. The experiments have led to a revival of interest in the foundations of quantum mechanics. QSD is used both as a theoretical and computational tool to study these systems, and as the basis of new approaches to the foundations. The book will interest graduate students and researchers in quantum mechanics and its applications, including quantum optics, quantum statistics, the philosophy of quantum theory and theoretical molecular biology.