Niels Bohr (1885-1962) was a Danish physicist who played a key role in the development of atomic theory and quantum mechanics, he was awarded the Nobel Prize for Physics in 1922. First published in 1924, this concise volume provides an English translation of a 1923 German language essay which appeared in the Zeitschrift für Physik journal. It concerns itself with the fundamental postulates of quantum theory, aiming to present the principles of the theory in such a way that their application appears free from contradiction. This book will be of value to anyone with an interest in Bohr's contribution to physics.