The flow of lava across the surface of the Earth and other planetary bodies is governed by the rheology (flow behavior) of the melt and the surface conditions present during eruption. High-resolution images of Venus, Mars, and Io (satellite of Jupiter) acquired during the past decade clearly show that lava flow is perhaps the most important process in the creation of rocky planetary surfaces. Detailed studies of the processes involved in lava flow are needed to better understand the evolution of our neighboring planets and to better mitigate volcanic hazards on the Earth.