Marcel Grossmann (April 9, 1878 – September 7, 1936) was a Swiss mathematician and a friend and classmate of Albert Einstein. Grossmann was a member of an old Swiss family from Zurich. His father managed a textile factory. He became a Professor of Mathematics at the Federal Polytechnic School in Zurich, today the ETH Zurich, specializing in descriptive geometry. Career In 1900 Grossmann graduated from the Federal Polytechnic School (ETH) and became an assistant to the geometer Wilhelm Fiedler. He continued to do research on non-Euclidean geometry and taught in high schools for the next seven years. In 1902, he earned his doctorate from the University of Zurich with the thesis Ueber die metrischen Eigenschaften kollinearer Gebilde (translated On the Metrical Properties of Collinear Structures) with Fiedler as advisor. In 1907, he was appointed full professor of descriptive geometry at the Federal Polytechnic School. As a professor of geometry, Grossmann organized summer courses for high school teachers. In 1910, he became one of the founders of the Swiss Mathematical Society. He was an Invited Speaker of the ICM in 1912 at Cambridge UK and in 1920 at Strasbourg. Collaborations with Albert Einstein Albert Einstein's friendship with Grossmann began with their school days in Zurich. Grossmann's careful and complete lecture notes at the Federal Polytechnic School proved to be a salvation for Einstein, who missed many lectures. Grossmann's father helped Einstein get his job at the Swiss Patent Office in Bern, and it was Grossmann who helped to conduct the negotiations to bring Einstein back from Prague as a professor of physics at the Zurich Polytechnic. Grossmann was an expert in differential geometry and tensor calculus; just the mathematical tools providing a proper mathematical framework for Einstein's work on gravity. Thus, it was natural that Einstein would enter into a scientific collaboration with Grossmann. It was Grossmann who emphasized the importance of a non-Euclidean geometry called Riemannian geometry (also elliptic geometry) to Einstein, which was a necessary step in the development of Einstein's general theory of relativity. Abraham Pais's book on Einstein suggests that Grossmann mentored Einstein in tensor theory as well. Grossmann introduced Einstein to the absolute differential calculus, started by Elwin Bruno Christoffel and fully developed by Gregorio Ricci-Curbastro and Tullio Levi-Civita. Grossmann facilitated Einstein's unique synthesis of mathematical and theoretical physics in what is still today considered the most elegant and powerful theory of gravity: the general theory