Small is strong - the yield stresses of thin metall23ic films with sub-micron thickness greatly exceed bulk values. While it is clear that this effect must be due to confinement effects on dislocation processes, the details of such mechanical finite size phenomena are complicated. Our recent in situ transmission electron microscopy studies combined with discrete dislocation dynamics simulations have shed some light on this problem. One conclusion is that the behavior of dislocations near interfaces affects the plasticity in small volumes in new ways which have not been foreseen by theoretical considerations so far. Another important aspect is the discovery of unexpected slip systems, which can be seen as indirect evidence for a constrained diffusional creep mechanism in polycrystalline films. Discrete dislocation simulations have allowed the operation of dislocation sources to be simulated under various boundary conditions. Based on these studies, we propose a new view of dislocation plasticity in thin films which, while being far from complete, seems to be more consistent with experimental observations and measurements.