The increasing use of high-performance fiber composites in critical structural applications has led to a need to predict the lifetimes of these materials in service environments. Such predictions require a basic understanding of (1) the microscopic deformation and failure processes of the composite, (2) the roles played by the fiber, epoxy matrix, and fiber-matrix interface in composite performance, and (3) the relations between the structure, the deformation and failure processes, and the mechanical response of these three elements and how those relations are modified by environmental factors.