The plastic behaviour of solids is reviewed in terms of defect motions, with the aim to include both crystalline and amorphous solids. The notion of defects is first extended to mean any localized atomic misfit, or density fluctuation, with a certain life time. Their localized character makes them easy to move with thermal aid, so that a strongly thermally activated plastic flow generally results. The thermodynamics of it is surveyed with an emphasis on activation parameters accessible to experiment (activation volume and energy) together with appropriate experimental methods (stress-relaxation test). The various possible stress-strain behaviours are classified depending on the type of defect motions they involve: anelasticity and internal friction, micro- or macroplasticity. Finally, the viscosity of materials under active atomic diffusion conditions is presented with the view to discuss from the same frame (the Eyring flow theory) both the crystal and the liquid viscosities, the origin of their large differences, and their connections with glass viscosities.