Simulation and experimental techniques have been developed to study the phase properties of a fluid confined between two closely-spaced walls and applied to investigate systems of charge-stabilized colloidal suspensions between repulsive smooth walls in contact with a reservoir. The phase behavior of confined suspensions is studied as a function of bulk particle volume fraction, surface charge, and wall separation. Complete crystallization within a fixed-size gap at sufficiently small wall separation occurs at bulk volume fractions well below the bulk freezing volume fraction. The simulations show a strong dependence of the freezing transition on a preferred wall separation corresponding to an integral number of layers. The simulation predictions match experimental observations at wall separations from five to ten particle diameters.