Both thermodynamic and kinetic effects can contribute to interphase formation. Thermodynamic features establish the intensity of the driving force and provide the `necessary' conditions for interphase formation, while kinetic factors such as diffusion, nucleation, and reaction of the components provide the `sufficient' conditions for interphase formation. The conviction that the interphase region plays a significant role in the performance of composite materials continues to motivate research on the behavior of the interphase and the exploration of approaches to tailoring the interphase. The mechanisms of interphase formation are discussed, along with the various aspects of interphase characterization.