Abstract The sections in this article are Introduction Equilibrium Properties of Solid–Solid Interfaces Structures of Grain Boundaries Structures of Interphase Boundaries Segregation to an Interface Interfacial Diffusion Forces for Interface Migration Capillary Forces Chemical Forces Mechanical Forces Elastic Strain Energy Plastic Response Frictional Forces Solute Drag Particle Pinning Examples The Massive Transformation Chemically‐Induced Grain Boundary Migration Discontinuous Precipitation Initiation of Discontinuous Precipitation Theories of Steady Cooperative Growth Experimental Observations Interface Migration in Multilayers Conclusions