On March 19, 2010, mathematics lost one of its leading geometric analysts, Johannes Jisse Duistermaat. At age sixty-seven he passed away, after a short illness following a renewed bout of lymphoma the doctors thought they had controlled. “Hans”, as Duistermaat was universally known among friends and colleagues, was not only a brilliant research mathematician and an inspiring teacher, but also an accomplished chess player and very fond of several physical sports. Hans dropped the subject of thermodynamics because the thesis had led to dissent between mathematicians and physicists at Utrecht University. Nevertheless, this topic exerted a decisive influence on his further development: in its study, Hans had encountered contact transformations. These he studied thoroughly by reading S. Lie, who had initiated their theory. In 1969–70 he spent one year in Lund, where L. Hörmander was developing the theory of Fourier integral operators; this class of operators contains partial differential operators as well as classical integral operators as special cases. Hans’s knowledge of the work of Lie turned out to be an important factor in the formulation of this theory. His mathematical reputation was then firmly established by a long joint article with Hörmander concerning applications of the theory to linear partial differential equations. In 1972 Duistermaat was appointed full professor at the Catholic University of Nijmegen, and in 1974 at Utrecht University, as the successor to Freudenthal