This text is an attempt to deal comprehensively with all the various theories and ideas which, during the course of the last two hundred years, have made modern architecture what it is today. Professor Collins has avoided the more usual archaeological approach and has concentrated on describing the various motives which have caused architects to select one form rather than another. Thus, since this is a history of ideas, it should be of interest not only to those who specialize in architecture, and have read the standard works of Hitchcock, Giedion, Pevsner, and Joedicke, but also to all those with a general interest in modern history and the philosophy of art. It explains what Revivalism, Rationalism, Eclecticism, and Functionalism really meant to those who practised them; it deals with the influence of the other arts and sciences on architectureal theory; in fact, it analyzes all the notions which are most commonly used in discussions about modern architecture, yet have implications frequently unsuspected or overlooked.