Thin film deposition techniques have been used to create high-quality polycrystalline multilayered films consisting of Al layers (6-100 nm thick) separated by thin (0.5-5 nm) layers of Al3Sc. The Al3Sc phase, which has the L1(2) crystal structure, is extraordinarily stable, has a lattice parameter only slightly larger than that of pure Al, and tends to form a coherent interface with Al. The plasticity of the Al/Al3Sc multilayers, tested by nanoindentation, is explained using simple models of dislocation motion where the dislocations are principally confined to the soft aluminum layers, but also where the anti-phase boundary (APB) energy plays an important role in determining the mechanical response.