Original published in 1926, this book presents a detailed account of quantum theory from its beginnings onwards. The text opens with a historical account of the quantum theory from its inception by Planck, and is followed by a full treatment of the dynamical theory evolved by Bohr and his school during the following ten years to explain the phenomena of line spectra. Some chapters are devoted to a general description of optical and X-ray spectra and their significance in the problem of the atomic structure of the elements. It closes with an account of work on the reaction of the atom to radiation fields. This book will be of value to anyone with an interest in quantum theory and the history of science.