In the 1950s, the distinguished theoretical physicist Wolfgang Pauli delivered a landmark series of lectures at the Swiss Federal Institute of Technology in Zurich. His comprehensive coverage of the fundamentals of classical and modern physics was painstakingly recorded not only by his students, but also by a number of collaborators whose carefully edited transcriptions resulted in a remarkable six-volume work. This volume, the fifth in the series, focuses on topics chosen by Pauli for their conceptual and historical interest: the probabilistic nature of quantum theory, the concept of spin, the problem of identical particles, and the relation of the statistics of rotational states of diatomic molecules to nuclear spin. Chapter headings include Wave Functions of Force-Free Particles, Description of a Particle in a Box and in Free Space, Particle in a Field of Force, More than One Particle, Eigenvalue Problems, Collision Processes, Angular Momentum and Spin, Identical Particles with Spin, and more. Originally published in 1973, the text remains an important resource thanks to Pauli's manner of presentation. As Victor F. Weisskopf notes in the Foreword to the series, Pauli's style is "commensurate to the greatness of its subject in its clarity and impact .… Pauli’s lectures show how physical ideas can be presented clearly and in good mathematical form, without being hidden in formalistic expertise." Alone or as part of the complete set, this volume represents a mathematically rigorous treatment that will be invaluable to individuals, as well as to libraries and other institutions.