Paul Ehrenfest was born in Vienna, where he started his studies as physicist with Ludwig Boltzmann, and later continued in Goettingen, Germany. He, along with his wife, Tatiana Afanashewa, carried out an investigation of kinetic theory. Their extensive article on statistical mechanics is still considered one of the classics on that field. From 1912 until his death Paul Ehrenfest was professor of physics at Leiden, Netherlands. His work was characterized by fundamental inquiry. He was an outstanding lecturer, and his enthusiasm for physics effected his students, although he was very strict about his demands for clarity and precision in expositions. Ehrenfest became a symbol of a period in physics characterized by two great advances, the quantum theory and the theory of relativity.