Publisher Summary This chapter discusses the diffusion through fiber/matrix interface. If there is no thermodynamic equilibrium between the fiber and the matrix, then physical–chemical interaction between them is inevitable. The consequence will be a structure of the interface region, which can contain substances with properties different from those of the component materials. In principle, the situation is quite clear but there exists some technical difficulties in describing all the details. The first difficulty arises because the phase diagrams of complex systems for elements presented in matrix and fiber materials are usually unknown. Secondly, the kinetics of the interaction in complex systems cannot be described exactly without special experiments. This means that it is difficult to evaluate interaction processes in a composite without making specimens and studying them. In fact, almost any matrix modification demands a special study. A simple case occurs when in the interfacial zone no new chemical compounds arise. The diffusion kinetics is governed by Fick's second law for the concentration that for planar case and a constant diffusion coefficient. High temperature treatments involved in the fabrication technology of metal- and ceramic-matrix composites as well as high temperature exposure at the service yield chemical reactions between matrix and reinforcement. It is suggested that a fiber-matrix combination desirable from the point of view of mechanical properties of the components is often characterized by a tendency to unwanted physical or chemical interaction. An obvious way to restrict this interaction is to introduce an interface layer to serve as a diffusion barrier, which also should provide a necessary value of the interface strength.