At the request of the Adams Fund Advisory Committee, and with the consent of the author, the following translation of Pro fessor Planck's Columbia Lectures was undertaken. It is hoped that the translation will be of service to many Of those inter ested in the development of theoretical physics who, in spite of the inevitable loss, prefer a translated text in English to an original text in German. Since the time Of the publication of the original text, some of the subjects treated, particularly that Of heat radiation, have received much attention, with the result that some of the points of view taken at that time have under gone considerable modifications. The author considers it de sirable, however, to have the translation conform to the original text, since the nature and extent of these modifications can best be appreciated by reference to the recent literature relat ing to the matters in question.
