Plastic deformation and strengthening of metals, a classic subject of physical metallurgy, is still a central theme of present-day materials research. This review focusses on two modern aspects of fundamental and practical interest: the mechanism of dispersion hardening at high temperatures, which allows the design of alloys operating close to their melting point; and the constraints on dislocation and diffusional deformation processes in metallic thin films, a potential reliability problem for micro-systems subjected to high internal stresses. The commonality lies in the importance of interfacial effects: the interaction of lattice dislocations with interfaces - between particle and matrix, or between film and substrate - controls the strengthening effect in both instances; diffusional creep occurs in both cases, but is again limited by interface effects. An attempt is made to summarize the current understanding of these phenomena with special emphasis on modelling and transmission electron microscopy results.