Atom diffusion is driven by a gradient of diffusion potential, which is the difference between the chemical potentials of atoms and vacancies. The diffusion potential near a surface is proportional to surface tension and curvature. In contrast, the diffusion potential along a grain boundary is influenced by the mechanical stress acting on it. The diffusion rate of atoms along surfaces and grain boundaries is orders of magnitude faster than that of bulk diffusion in crystals. Matter transport also occurs through evaporation and condensation to and from the vapor phase, as well as through solution and precipitation to and from the liquid phase. Diffusion in ionic crystals involves the point defects of more than one charged species and is influenced by dopant, impurity, and non-stoichiometry.