We develop a numerical method for examining complex morphologies in thin films of block copolymer using self-consistent;field theory. Applying the method to confined films of symmetric diblock copolymer, we evaluate the stability of parallel, perpendicular, and mixed lamellar phases. In general, lamellar domains formed by the diblocks are oriented parallel to the film by surface fields. However, their orientation can flip to perpendicular when the natural period of the lamellae is incommensurate with the film thickness. Experiments and Monte Carlo simulations have indicated that mixed lamellar phases may also occur, but for symmetric diblocks, we find these phases to be slightly unstable relative to perpendicular lamellae. Nevertheless, just a small asymmetry in the molecule stabilizes a mixed lamellar phase. Although our work focuses on confined films, we do discuss the behavior that results when films are unconfined.