The increasing deployment of composite laminates in engineering applications, many of which are being designed as the primary load-carrying parts in hostile environments, has given greater attention to the long-term behavior of composite components. Accordingly, there is an urgent need in the engineering community for a predictive tool of the durability, reliability, damage tolerance, and safety of composite systems so that it is possible for designers to certify structures for service, to predict the consequences of material selection and design specifications on long-term performance, and to reduce the need for costly tests. As a result, the Materials Response Group at Virginia Tech has devoted a great deal of research to composite fatigue phenomena due to cyclic loading and environmental effects (temperature, moisture, physical aging, etc.). Our efforts have led to the development of a Performance Simulation Code for Material Systems, MRLife, (current release version 12, i.e. MRLife12). The objective of the performance simulation approach in MRLife^TM is to provide a method of bringing together the material data, analytical tools, and understanding of composite fatigue failure to predict the long-term mechanical behavior as a function of the material system design parameters and the microdamage processes.