The disk instability model suggests that gas giant planets form directly as a result of (local) gravitational instabilities in the protoplanetary disk. Numerical simulations have shown that clumps of about one to several Jupiter masses can form in sufficiently gravitationally unstable disks. Protoplanetary disks can become gravitationally unstable if they are sufficiently cool and/or massive. The condition for gravitational instability can be determined from the value of the Toomre parameter Q. Protoplanetary disks will be unstable for Q < ~1. Once a local instability occurs, a gravitationally bound sub-condensation region (clump) is created. If the cooling time in this region of the disk is shorter than (or comparable to) the dynamical time, the fragment contracts, and eventually evolves to become a giant planet.