The potential new markets for polymermatrix fiber composites (PMCs) are in the commercial aircraft, transportation, machinery, marine, and public works industries. A common requirement for many of the envisioned applications is the need to withstand structural loading for extended periods of time. Although good design using PMCs usually implies exploiting the high strength and stiffness properties of the fiber reinforcement, this article demonstrates that the matrix and interface properties play primary roles in the long-term durability of these materials.