THEORETICAL physics has lost one of its ablest and most enthusiastic exponents through the death on September 25 last of Paul Ehrenfest, professor of theoretical physics in the University of Leyden. Born in Vienna in 1880, he studied first under Boltzmann. From Vienna he went to Gottingen; there he made the acquaintance of his future wife (Mrs. T. Ehrenfest-Afanashewa). In collaboration with Mrs. Ehrenfest, he undertook a thorough critical investigation of the principles underlying the kinetic theory of matter, a problem which had fascinated him ever since his Vienna days. The elucidation of certain paradoxes in Boltzmann's theorem (1907) and the extensive article on statistical mechanics in the “Enzyklopadie der mathematischen Wissenschaften” (1912) testify to the fertility of their combined activity. The latter work was written in Russia, where the Ehrenfests lived for several years.