Volcanism appears to have been a major process in the formation of surface materials on each of the silicate-dominated planets and satellites in the solar system at some stage in its development. Observations of the types and extents (in space and time) of volcanic activity on a planetary body can provide important information on the interior structure and chemistry, and on the near-surface environmental conditions. Attempts to develop deterministic models of volcanic mechanisms which are sufficiently general to be applicable to eruptions of a given kind on any planet have been a major spur to understanding the geological processes involved. We are currently able to give fairly complete descriptions of some eruption processes, in the forms in which they have operated on several planets.