An analysis of strain relaxation in thin films on substrates indicates that, at temperatures below 0.5Tm this phenomenon can adequately be described in terms of three mechanisms, namely diffusion creep, grain boundary sliding and plastic deformation through dislocation glide. The efficiency of the grain boundaries as sources and sinks for vacancies is an important factor in such strain-relaxation. Numerical solutions of the applicable difference equations with appropriate boundary conditions, set up to describe strain-relaxation in thin films on substrates, are in fair agreement with available experimental data. These solutions also indicate that a planar elastic strain of about 2-3 × 10-3, and a dislocation density of the order of 103 cm/cm3 or higher, may usually be expected in thin films even after annealing treatments. © 1974.