The achievement of the potential of composites for demanding applications where high specific strength and stiffness are required together with useful toughness is often limited by the available materials processing capabilities to develop specific reinforcement-matrix interface characteristics. Similarly, the elevated temperature stability of advanced composites is dominated by the behavior of internal interfaces. In order to develop effective processing strategies and stable composite designs, it is essential to consider the relevant phase diagrams which for most composites are at least of ternary order. On the basis of these diagrams, it is possible to select compositions of phases which possess desirable properties. In addition to phase diagram data, kinetic data such as the interdiffusion pathway and rates are required to understand and to control the possible interfacial chemical reactions in a composite system. From this basis, reinforcement coatings and barrier layers can be developed to control reactions and allow for in-service lifetime analysis. With solidification processing of composites, melt-reinforcement interactions involved in wetting, solidification reactions, and matrix microstructure evolution can also be evaluated in terms of the phase equilibria and kinetic pathways. Some applications of this approach have been demonstrated in the development of Ti- and A1-based composite systems.