Abstract
This paper provides an overview of our state of knowledge about comets prior to the Rosetta mission encounter. Starting with the historical perspective, this paper discusses the development of comet science up to the modern era of space exploration. The extent to which comets are tracers of solar system formation processes or preserve pristine interstellar material has been investigated for over four decades. There is increasing evidence that in contrast with the distinct dynamical comet reservoirs we see today, comet formation regions strongly overlapped in the protoplanetary disc and there was significant migration of material in the disc during the epoch of comet formation. Comet nuclei are now known to be very low-density highly porous bodies, with very low thermal inertia, and have a range of sizes which exhibit a deficiency of very small bodies. The low thermal inertia suggests that comets may preserve pristine materials close to the surface, and that this might be accessible to sample return missions.This article is part of the themed issue 'Cometary science after Rosetta'.