This book provides the reader with an explanation of the origin and establishment of quantum mechanics together with a descriptive survey of developments up to the present day. The mathematics is presented in a digestible form yet following the original approach.This second edition presents two new chapters to supplement and extend the first edition material. “Interpretations of Quantum Mechanics” surveys a wide range of current topics, including the multiverse, 't Hooft's ideas for a deterministic local field theory, a summary of the de Broglie-Bohm pilot-wave theory and Anthony Valentini's development of it, and speculative concluding comments on the way ahead.“A Reflective Interlude” looks in more detail than hitherto at the origin and early years of wave-particle duality, with emphasis on trying to discover, as far as possible, what was the physical reality implied by de Broglie's work as it progressed.Appendices include useful reminder notes on associated background topics, with a new appendix “Planck Units”.With references to the original works, to reviews and useful bibliographies, the reader is uniquely well-equipped to delve further into the subject.In addition to its importance for those studying physics, this book is also intended for those studying the history of science.