  This article reviews the biography of the Swiss mathematician Marcel
Grossmann (1878-1936) and his contributions to the emergence of the general
theory of relativity. The first part is his biography, while the second part
reviews his collaboration with Einstein in Zurich which resulted in the
Einstein-Grossmann theory of 1913. This theory is a precursor version of the
final theory of general relativity with all the ingredients of that theory
except for the correct gravitational field equations. Their collaboration is
analyzed in some detail with a focus on the question of exactly what role
Grossmann played in it.
