The study of the influence of moisture on the properties of polymer composite materials has been drawing attention to this day. The negative impact of moisture on the strength and performance properties is the factor, the forecast of which is necessary to ensure the reliability and durability of polymer composite materials. This review considers methods for determining moisture absorption and diffusion in polymer materials with fibrous (woven) fillers based on epoxy resin; current domestic and foreign standards for determining moisture content in polymer composite materials; methods for determining and assessing the effect of absorbed moisture on polymer composite materials, including conditions of operation and manufacturing processes (curing state). The data on the influence of climatic factors on polymer composite materials for the last couple of years are briefly presented. The review will be useful for researchers dealing with the problem of moisture absorption and water absorption by composite materials in the form of a generalization of the accumulated knowledge on this topic in order to analyze the long-term behavior of polymer materials.