Scott Tremaine conducts important work on planetary rings and discs, and in theoretical astrophysics. The kinetic theory of rings and particles in both dilute and dense limits is due to him and his collaborators and he has studied numerically the intermediate case. With Peter Goldreich, he predicted density waves in Saturn’s rings and shepherd satellites of thin rings since observed by spacecraft in the ring systems of Saturn and Uranus. He demonstrated that tides, not panning stars, are the primary creators of ‘new’ comets and in consequence modified our concept of the extent of Oort’s proto-cometary cloud. With James Binney, he has written an outstanding book on galactic dynamics, a field to which he has made vital contributions. His considerations of phase space put severe constraints on the idea that neutrinos with rest mass are a major constituent of the Universe. He has derived new tests of the equivalence principle for both neutrinos and photons. Scott was founding Director of the Canadian Institute for Theoretical Astrophysics to which he drew a most able staff.