Future historians of science will look back on the arrivals of the Voyager spacecraft at the Jupiter system in 1979 as very significant events, primarily because of the remarkable diversity of new phenomena discovered on the Galilean satellites. In a short time period, there was almost a doubling of the number of solid “planetary” bodies for which substantial scientific analysis is possible. Scientists were introduced to the novelties of tidally heated bodies and of ice tectonics. From a more fundamental point of view, there is much to be learned about the origin and evolution of planets and their environments from analysis of the satellite systems. Any doubts about the importance of studying planetary satellites must have been dispelled by the Voyager missions.