Abstract: Mechanical properties depend strongly on size; while this principle has long been exploited in the microstructural design of materials, it is nowadays possible to structure materials in the form of films with thicknesses down to nanometer dimensions. A question of both fundamental and practical importance is how the mechanisms of deformation scale with size and what the limiting strength values are. Research over recent years has shown that below the "sound barrier" of about one micrometer film thickness the plastic behavior, fatigue resistance and other properties are dramatically modified. This is due to the interaction of lattice defects with the boundaries and interfaces in nano-scale materials. Also, the substrate constraint can give rise to new deformation mechanisms, such as dislocation glide on unexpected planes (termed "parallel glide"). In this presentation, an overview of our recent results on fcc metals will be given. Particular emphasis will be placed on in-situ transmission electron microscopy and a newly developed synchrotron technique for measuring stress-strain curves of ultra-thin films.