John William Scott (‘Ian’) Cassels. 11 July 1922—27 July 2015

J. W. S. Cassels was a distinguished number theorist, who was Sadleirian Professor in Cambridge from 1967 to 1984. He worked in many areas of the subject. Perhaps his greatest contribution was concerning Elliptic Curves, much of which is described in a series of eight ground-breaking papers. He also made significant contributions to Diophantine Approximation, Geometry of Numbers and Quadratic Forms, as well as writing influential textbooks on all of these areas. Furthermore, he was an early exponent of the use of computers as an experimental tool in number theory.