The observed properties of giant planets, models of their evolution, and observations of protoplanetary disks all provide constraints on the formation of gas giant planets. The four largest planets in our Solar System contain considerable quantities of hydrogen and helium, which could not have condensed into solid planetesimals within the protoplanetary disk. The preponderance of evidence supports the core nucleated gas accretion model of formation of the giant planets. According to this model, giant planets begin their growth by the accumulation of small solid bodies, as do terrestrial planets. However, unlike terrestrial planets, the growing giant planet cores become massive enough that they are able to accumulate substantial amounts of gas before the protoplanetary disk dissipates. During this talk, Dr. Lissauer will present the first models of giant planet formation that account for both the planet's internal energy budget and gas flows within the protoplanetary disk.