As part of a general study of the behavior of materials with temperature‐dependent properties, simple models of a viscoelastic solid are considered here. Ideal experiments at constant stress and constant rate of deformation are examined. The analysis leads to yield and fracture criteria which in the usual theory are regarded as experimentally determined quantities. Rate of strain and size effects are also deduced and a phenomenon resembling strain hardening can develop. The treatment is independent of, but complementary to, the atomic scale theories of the deformation of solids. The temperature coefficient of viscosity which is introduced into the continuum theory here, is related to the energy of activation for the flow process which is accessible to the atomic scale theory. Knowledge of the temperature field in which motions of the atoms occur could improve the predictions based on dislocation theory.