The recent discoveries of extrasolar giant planets, coupled with refined models of the compositions of Jupiter and Saturn, prompt a reexamination of theories of giant planet formation. An alternative to the favored core accretion hypothesis is examined here; gravitational instability in the outer solar nebula leading to giant planet formation. Three-dimensional hydrodynamic calculations of protoplanetary disks show that giant gaseous protoplanets can form with locally isothermal or adiabatic disk thermodynamics. Gravitational instability appears to be capable of forming giant planets with modest cores of ice and rock faster than the core accretion mechanism can.