The surface energy is crucial in sintering; however, a distinction exists between surface energy and surface tension (surface stress) in crystalline solids. Since the surface energy of a crystal is anisotropic and varies with temperature, the equilibrium crystal shape transitions from faceted to rounded forms, becoming spherical at temperatures above the roughening transition. The grain boundary energy is also anisotropic, depending on boundary orientations defined by five independent parameters. Grain boundary kinetics at the atomic scale is characterized by grain boundary migration (conservative motion) and rigid body motion (non-conservative motion). The grain boundary structure is significantly altered by segregation and the formation of grain boundary complexion, which includes intergranular glassy films.