Hygrothermal behavior of cured composite materials relates to the combined and commonly synergistic effects of moisture absorption and temperature on various physical, chemical and mechanical properties. This article focuses on the influence of resins or matrices, reinforcements, processing, and diffusion on hygrothermal behavior of polymer-matrix composites and provides an outline on general considerations in assessing them. It discusses the hygrothermal testing and conditioning of polymer-matrix composites to assess fundamental hygrothermal behavior. The article provides information on the accelerated aging process for understanding the degradation mechanisms and failure modes in composites. It also describes the effect of moisture absorption on mechanical properties of polymer-matrix composites.