Jupiter's four Galilean moons — Io, Europa, Ganymede, and Callisto — were discovered by Galileo in 1610 and remain the archetype of a giant-planet satellite system. Io is the most volcanically active body in the Solar System, heated by tidal flexing from Jupiter and resonant tugs from Europa and Ganymede. Europa hides a subsurface ocean beneath its icy crust, making it a prime target in the search for extraterrestrial life. Ganymede, the largest moon in the Solar System, generates its own magnetic field. Callisto, the outermost, bears the most ancient, crater-saturated surface. Their 4:2:1 orbital resonance is one of the clearest examples of tidal evolution shaping a moon system.