The kinetics of solid/solid interfaces controls in part the course of heterogeneous reactions in the solid state. The essential situations are defined and basic formalisms are summarized. In addition to the role interfaces play as resistances for transport across them, they offer high diffusivity paths laterally, representing two-dimensional reaction media. Experimental examples of kinetic phenomena are static and moving boundaries, including exchange fluxes, boundary-controlled solid state reactions, interface morphology, nonlinear phenomena connected with interfaces and reactions in and at boundaries. The case of a moving semicoherent boundary AX/BY is modeled quantitatively, and experiments in the system AgBr/KBr are reported.