Comets are surviving members of a formerly vast distribution of solid bodies that formed in the cold regions of the solar nebula. Cometary bodies escaped incorporation into planets and ejection from the solar system and most have been stored in two distant reservoirs, the Oort cloud and the Kuiper belt, for most of the age of the solar system. The past decade has seen great advances in the understanding of comets, but they still remain somewhat mysterious and enigmatic bodies from the fringes of the solar system. The distinction between comets and asteroids has become clouded with the discovery of active bodies in the asteroid belt and the finding that comets contain ‘common meteoritic solids.’