The use of micromechanics equations for moisture diffusivity shows that the in situ diffusivity is slightly lower than the bulk diffusivity for matrix resins, thereby indicating absence of any matrix damage in virgin composites. When exposed to hygrothermal environments, however, composites undergo degradation which manifests itself in anomalous moisture diffusion behavior and reduced structural performance. The hygrothermal degradation is the result of matrix plasticization, microvoid formation, and microcracking. The time dependence of plasticization as well as the tensile stress resulting from steep moisture gradient is responsible for the damage induced by thermal spiking of wet composites. Swelling of neat resins is frequently less than predicted by the volume additivity. A simple micromechanics analysis provides a good estimate of composite swelling strain from resin properties. The bilinearity and the hysteresis observed in relations between swelling and moisture content are attributed to the existence of a threshold moisture concentration below which swelling is negligible. Relaxation of residual stresses is a long-term process under nonhostile environments. However, it is accelerated considerably around and above the glass transition temperature. The fast stress relaxation can change the transverse ply stress from compressive to tensile after thermal spiking, thereby inducing ply cracking and accelerating the subsequent moisture absorption. How residual stresses can affect ply cracking and delamination is shown through a fracture mechanics analysis.