Many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells. This article first reviews how such oceans are detected. We then discuss how they are maintained, when they formed, and what the oceans' likely characteristics are. We focus in particular on Europa, Ganymede, Callisto, Titan, and Enceladus, bodies for which there is direct evidence of subsurface oceans. We also consider candidate ocean worlds such as Pluto and Triton.