Chapter 6 describes theoretical models for the formation of giant planets, and relevant observational constraints from the Solar System. The core accretion theory for giant planet formation is introduced, including the equations describing planetary envelope structure, the concept of a critical core mass, and illustrative evolutionary tracks for giant planet growth. Current knowledge of the internal structure of Jupiter, based on measurements of the gravitational field, is summarized. The conditions under which a massive gas disk becomes unstable and fragments are described, together with the likely outcome of disk instability.