Understanding the formation of giant planets with substantial gaseous envelopes forces us to confront once again the physics of the gas within the protoplanetary disk. Unlike the case of terrestrial planet formation, two qualitatively different theories have been proposed to account for the formation of massive planets. In the core accretion theory of giant planet formation, the acquisition of a massive envelope of gas is the final act of a story that begins with the formation of a core of rock and ice via the identical processes that we discussed in the context of terrestrial planet formation. The time scale for giant planet formation in this model – and to a large extent its viability – hinges on how quickly the core can be assembled and on how rapidly the gas in the envelope can cool and accrete on to the core. In the competing disk instability theory, giant planets form promptly via the gravitational fragmentation of an unstable protoplanetary disk – a purely gaseous analog of the Goldreich–Ward mechanism for planetesimal formation that we discussed in Chapter 4. Fragmentation turns out to require that the disk be able to cool on a relatively short time scale that is comparable to the orbital time scale, and whether these conditions are realized within disks is the main theoretical issue that remains unresolved. Drawing on our prior results on gravitational instabilities in disks and on terrestrial planet formation, the goal in this chapter is to describe the physical principles behind both models and to provide a summary of some of the relevant observational constraints.