Macroscopic densification behavior can be accurately predicted from knowledge of microstructure using periodic equilibrium pore structure models. The sintering stress is defined as the volume derivative of the total energy based on the principle of equilibrium thermodynamics. It is also defined in terms of kinetics using the sintering force. Although both methods yield the same values for equilibrium structures, the kinetic method is advantageous since it can be applied to the nonequilibrium sintering process. It is demonstrated how viscosity tensors are influenced by detailed local structure, the ratio of grain boundary energy to surface energy, grain boundary sliding, and anisotropic structures. The bulk viscosity and shear viscosity of an isotropic system are derived by averaging the viscosity tensor of a cubic structure across all directions.