Composite materials have been the most significant revolution in structural mechanics since 1960. There is a general lack of data on the long-term performance of these materials. The objective of this thesis is to develop a methodology, based on mechanistic models, for predicting the long-term durability of fiber reinforced polymer (FRP) composite materials. Accelerated testing simulating the composites applications in various constructions and fields has been used to validate it. The research developed a basic predictive tool using the Arrhenius principles adapted to the TTS (Time Temperature Superposition) and measuring degradation of carbon fiber epoxy composite under UV (Ultra Violet) and hygrothermal exposure. The tests captured the synergistic effects of field exposure and extreme temperatures, viz., hot/dry, hot/wet, cold/dry, and cold/wet. The feasibility to predict capacity loss from changes at the molecular level, such as hydrolysis, micro- cracks, and UV induced polymer chain scission, has been established using a unique finite-element (FE) based multi-scale, multi-mechanism degradation model.