Following the highly successful Galileo Mission to Jupiter, a deeper, more complete, yet ultimately challenging picture of the major moons Io, Europa, Ganymede, and Callisto has emerged. This talk will assay the interior structures, compositions, and thermal evolutions of all four bodies, highlighting the roles of volcanism, internal oceans ("aquaspheres"), and core formation. Speculations will be offered regarding Ganymede's magnetic dynamo and Europa's astrobiological potential.