To understand the physics of high-temperature deformation of crystals, we first need to describe the rheological behaviour of the solid in terms of mechanical and physical variables (stress, strain, temperature, pressure …). The description is embodied in constitutive equations, obtained by means of mechanical tests. In the present chapter, we summarily introduce the fundamental notions needed: stress, strain, and the various rheological constitutive equations. At high temperatures many materials flow viscously and viscous behaviour is therefore especially important. The principal methods of mechanical testing – creep at constant stress, deformation at constant strain-rate and stress relaxation – are presented and compared. The role of the variables in the constitutive equation is discussed: time, a special kinematic variable, explicitly appearing in transient creep only; strain, usually not a good variable, except when it coincides with the structural variables; strain-rate and stress. Minimum creep-rate, steady-state creep-rate and constant-structure creep-rate generally correspond to different conditions and must not be confused. We are concerned here with uniform deformation, but it may be useful to consider briefly the criteria for non-uniformity (i.e. localization) of deformation. Shear localization is a plastic instability manifesting itself as a stress drop on stress–strain curves.