This thesis represents the long-term behavior of fiber reinforced polymer (FRP) materials for structural applications. FRP composites consist of high-strength fibers embedded in an epoxy resin. The long-term investigations include i) pultruded glass FRP (GFRP) beams subjected to sustained loads and cold temperature and ii) reinforced concrete beams strengthened with near-surface mounted (NSM) carbon FRP (CFRP) strips. For the first phase, test parameters include the variation of sustained intensities and temperature. The flexural behavior of the long-term beams is studied through a combined experimental and numerical approach, including load-carrying capacity, failure mode, creep response, and material degradation. Some material parameters that are crucial for practical applications are suggested using a regression analysis. A finite element model is developed to predict the behavior of GFRP beams. An analytical model is also proposed to estimate the long-term behavior of GFRP composites for structural applications. For the second phase, test parameters include the variation of sustained intensities, CFRP strengthening schemes and bonding agents. The short-term beams are loaded both monotonically and cyclically whereas the long-term beams are loaded only monotonically. The flexural behavior of all beams is studied through an experimental investigation, including load-carrying capacity, failure mode, creep response, and material degradation.