Daniel "Dan" Quillen died on April 30, 2011.
Quillen received a Fields Medal at the International Congress of Mathematicians held in Helsinki in 1978. He received the award as the principal architect of the higher algebraic K-theory in 1972, a new tool that successfully used geometric and topological methods and ideas to formulate and solve major problems in algebra, particularly ring theory and module theory.
From 1984 to 2006 he was the Waynflete Professor of Pure Mathematics at Magdalen College, Oxford.
More information: http://en.wikipedia.org/wiki/Daniel_Quillen