Crystalline thin films are generally metastable in the as-deposited state and will undergo solid state dewetting if they are heated to temperatures leading to sufficiently high atomic mobilities. Dewetting is driven by surface energy minimization and usually proceeds by curvature-driven surface self-diffusion. The driving energy (a function of the surface-to-volume ratio) is high and the diffusion distances are short in especially thin films or small patterned structures, so dewetting can occur well below the film’s melting temperature. When unpatterned polycrystalline films dewet on flat substrates, the dewetting process is strongly influenced by the film’s gran structure. While the resulting islands have a characteristic length scale that roughly scales with the film thickness, the island spacings and sizes are broadly distributed. However, topographic pre-patterning of the substrate surface can serve to template the solid-state dewetting process and produce ordered arrays of monodispersed and crystallographically aligned islands. Single-crystal films dewet to form more regular structures that are strongly influenced by the crystallographic alignment of the film. In this case, the dewetting process can be templated through prepatterning of the film itself. Selection of different specific initial patterns allows independent study of the various mechanisms that control structure evolution during solid-state dewetting. These include fingering instabilities, edge faceting, corner instabilities, pinch-off processes, and Rayleigh-like instabilities. Studies of dewetting of pre-patterned single crystal films provide data for development and testing of models that capture the effects of surface-energy anisotropy and facet formation on shape evolution of crystalline solids.