Paul Ehrenfest was born on January 18, 1880 in Vienna, Austria. He attended the Technische Hochschule in Vienna and then the University of Göttingen. He received the doctoral degree in physics from the University of Vienna in 1904. Ehrenfest met Albert Einstein in 1912, beginning a friendship that would last for the rest of his life. The same year, he accepted a position of professor of physics at the University of Leiden, in the Netherlands, in part because of Einstein's recommendation.
Ehrenfest is best known for his contributions to quantum theory, particularly the application to rotating bodies. He is important in the history of probability because of his diffusion models, such as the Ehrenfest chains, developed to give a statistical interpretation to the second law of thermodynamics.
Ehrenfest committed suicide on September 25, 1933 in Leiden.