[en] We have studied the transition from two to three dimensions in a low temperature weakly interacting ⁶Li Fermi gas. Below a critical atom number N_2D only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two dimensional. Above N_2D the Fermi gas enters the quasi-2D regime where shell structure associated with the filling of individual transverse oscillator states is apparent. This dimensional crossover is demonstrated through measurements of the cloud size and aspect ratio versus atom number.