Although he spent much of his professional life in industry, Hendrik Casimir was one of the great Dutch theoretical physicists. Casimir made many contributions to science during his years in research from 1931 to 1950. These contributions include: pure mathematics, Lie groups (1931); hyperfine structure, calculation of nuclear quadrupole moments, (1935); low temperature physics, magnetism, thermodynamics of superconductors, paramagnetic relaxation (1935–1942); applications of Onsager's theory of irreversible phenomena (1942–1950). He helped found the European Physical Society and became its president from 1972 till 1975. In 1979 he was one of the key speakers at CERN's 25th anniversary celebrations. In 1946 he became member of the Royal Netherlands Academy of Arts and Sciences.