The largest and most important hydropower dams in Sweden are concrete buttress dams. These consist of a large number of concrete monoliths formed by a frontplate with a supporting buttress. Cracks have been observed in some of these dams, and these may affect their safety in the long term. As an important tool for the condition assessment of such dams, a finite element model based on nonlinear fracture mechanics and plasticity theory has been used to study crack development in a buttress dam. The combined effects of restrained thermal displacements and loads due to water were studied. The development of cracks due to seasonal temperature variations was simulated, especially the effect of an insulating wall installed some years after the completion of the dam. Thermal stresses, in combination with the load caused by water, were shown to be the reason for cracking. The addition of an insulating wall greatly contributed to the development of cracks in the buttress.