Icy moons are believed to be a common class of planetoids that have a surface mostly of ice, possibly with an ocean under the ice, and possibly including a rocky core of silicate or metallic rocks. The prototype of this class of object is Europa.

Icy moons warmed by tides may be the most common type of object to have liquid water, and thus the type of object most likely to have water-based life.

Also some icy moons exhibit cryovolcanism.