  Gas giant planets play a fundamental role in shaping the orbital architecture
of planetary systems and in affecting the delivery of volatile materials to
terrestrial planets in the habitable zones. Current theories of gas giant
planet formation rely on either of two mechanisms: the core accretion model and
the disk instability model. In this chapter, we describe the essential
principles upon which these models are built and discuss the successes and
limitations of each model in explaining observational data of giant planets
orbiting the Sun and other stars.
