Fiber-reinforced polymer composites have extensive applications due to their high specific strength, improved product performance, low maintenance and design flexibility. However, moisture absorbed by polymer composites during the service life plays a detrimental role in both the integrity and durability of composite structure. It is essential to understand the moisture diffusion behavior and induced damage in polymer matrix composites under varying hygrothermal conditions. In Part I, the moisture diffusion characteristics in hybrid composites using moisture concentration-dependent diffusion method have been investigated. Also, a multi-stage diffusion model was proposed to explain the deviation of moisture diffusion behavior for sandwich composites from classical Fick's law using a time-dependent diffusivity scheme. User-defined subroutines were developed to implement these methods into commercial finite element code. To validate the simulation results, an open-edge moisture diffusion experiment was conducted for sandwich composites composed of woven E-glass fiber-reinforced polyurethane (PU) face sheets and a closed-cell rigid PU foam core. In Part II, the behavior of moisture diffusion and its effects on the mechanical properties of carbon/bismaleimide composites exposed to seawater conditioning at elevated temperatures were investigated. The degradation of mechanical properties due to hygrothermal aging was assessed by conducting short beam shear test and flexural test at three immersion time points. In Part III, the effect of moisture on mechanical performance of PU sandwich composites was investigated. Mechanical property degradation due to moisture absorption was evaluated by conducting compression test of the foam core, flexural test of the laminates, and double cantilever beam Mode-I interfacial fracture test of sandwich composites.