Polymer composites represent materials of great and of continuously growing importance. Their potential for application appears to be limitless. They have been the subject of numerous studies both at academic and industrial levels. Much progress has been made in the incisive formulation of composites; sophisticated methods of property evaluation have been developed in the past decade and many, largely empirical solutions have been proposed to resolve the problem of their long-term performance under typical conditions of use (i. e. the use of silane or titane coupling agents to enhance adhesion within composite materials). Assuredly one of the most essential factors in the performance of these systems is the condition of the interface and interphase among the constituents of a given system. It has become clear that it is the interface/interphase, and the interactions which take place in this part of a system, which determine to a significant degree the initial properties of the material. In order to achieve leadership in the formulation and application of polymer composites, it is evident that in depth understanding of interfacial and interphase phenomena becomes a prerequisite.