Ernst Mach (1838-1916) is chiefly famous on two counts: for his brilliant experimental work on shock waves (in recognition of which the Mach number is named after him) and for his numerous critical historical studies of the development of physics. It was through these studies that Mach became an extremely influential and controversial figure in the physics of the twentieth century. This explains why the present work, a translation into English of his Die Principien der Wärmelehre, which was first published in 1896, now appears as Volume 17 of the Vienna Circle Collection published by Reidel. (The Vienna Circle was a discussion group of philosophically interested scholars who met weekly from 1925 to 1936 at the University of Vienna. Mach was regarded as one of their precursors.)