Abstract Levels of intrinsic stress in thin films deposited using energetic condensation of ions and/or atoms are known to depend strongly on the energy of the depositing species. At low energies (up to a few eV) thin films are often observed to have a large number of voids and show tensile stress. As the energy increases, the stress is observed to rise to a maximum at an intermediate energy—usually at a few tens to a few hundreds of eV. Thereafter as the energy is increased further, the intrinsic stress is found to decrease. Variations on this “universal” behaviour occur when species of two quite different energies are deposited whereupon the higher of the two energies seems to determine the level of intrinsic stress even when the fraction of ions at that energy is comparatively small. This finding has led to the use of the plasma immersion ion implantation (PIII) method during film deposition to produce coatings with low stress. Although 98–99% of the ions impacting the substrate have energies in the range which produces very high intrinsic stress, the intrinsic stress in the film is determined predominantly by the 1–2% of ions with high energy (kV and above). Recent experiments show that stress relief can be achieved in films deposited with high levels of stress by post-deposition ion implantation with a non-condensing species, such as argon. We carried out in-situ ellipsometric studies during the implantation and relaxation process, and found that the thickness of the film increased. The increase in thickness in carbon was consistent with a transition of material in the treated volume from sp 3 rich to sp 2 rich chemical bonding. Atomistic simulations also indicate that the film expands by raising the surface when stress is relieved by energetic bombardment. Experiments with a variety of materials show that a phase transition or change in preferred orientation occurs during the stress relief process. This paper will explore the physical processes involved in determining stress levels in thin films using experimental results together with atomistic simulation. Experimental results of intrinsic stress as a function of ion energy when dc bias is applied, so that a high proportion of the depositing species are affected, are compared to results of PIII&D processes where only a small proportion of ions are affected by the applied bias. Mechanisms and simple models of stress generation and relief are proposed and compared with experiment. The carbon system is examined in detail and the effects of thermal annealing (also found to reduce stress but with different effects on microstructure) are compared with that of high energy ion bombardment. The differences are explained in terms of the model presented.