The discovery of numerous extra-terrestrial volcanoes, including active ones, has stretched our traditional definition of what a volcano is. We now know that the nature of volcanism is highly variable over the solar system, and the traditional definition of a volcano as defined for Earth needs to be modified and expanded to include processes such as cryovolcanism, in which aqueous mixtures are erupted from the interior to the surface. In this chapter, we review past volcanism on the Moon, Mercury, and Mars, active volcanism on Io, and cryovolcanism in the moons of the outer solar system. We suggest the following definition that encompasses the different forms of volcanic activity seen in other worlds: A volcano is an opening on a planet or moon's surface from which magma, as defined for that planetary body, and/or magmatic gas is erupted .