Micro- and nano-structural features within thin films and multilayers control their resistance to plastic deformation. In the present work, we investigate the relationship between microstructure and strength of multilayered thin films composed of Al and Sc. The Al/Sc system is currently receiving a great deal of attention due to the significant strengthening effect of Sc in Al alloys. Thin film deposition processes allow the synthesis of multilayered thin films of arbitrary compositions. We have used this to advantage to create high-quality polycrystalline multilayered films consisting of soft Al layers (6-100 nm thick) separated by thin (0.5-5 nm) layers of Al3Sc. We have produced volume fractions of Al3Sc far in excess of those possible by traditional casting techniques. Nanoindentation and wafer curvature tests have revealed that the multilayer films are extremely strong, with increases in hardness as much as 3-4 times the rule of mixtures value. Characterization, via FIB, High Resolution TEM and X-ray diffraction, has shown these layers to be coherent, providing a simple (and ideal) test system for investigating the strengthening behavior of metal multilayers. We find that the strength of these materials increases markedly with decreasing bilayer period, as expected, but these increases are smaller at very small bilayer periods. We present a model based on misfit dislocation creation and propagation in coherent multilayers as an attempt to explain the observed strengthening behavior.