The aging of polymer-composites is a ubiquitous problem that leads to the degradation of mechanical properties, reducing the service life of an engineered structure, and potentialising premature, catastrophic modes of failure. Polymer-composites used in moist or aqueous environments are subject to moisture influenced changes that affect their physical, chemical and mechanical properties. The coupled problem of polymer-composites aging within an aqueous environment is currently seeing a surge in research efforts. This is partly due to that materials used in renewable energy structures, such as tidal turbine blades, are now high-priority concerns and there is mounting societal pressure for the development of clean energy technology. The coupling of aging and water ingress in polymer-composites is not a trivial subject and is a very slow process, but as a consequence of clean energy technology concerns, there is an ever growing impetus towards the research of exacerbated rates of water aging by the integration of a third influence, heat. Heat is a means by which the rate of aging can be magnified and this combination of heat induced aging with water ingress, termed hygrothermal aging, is the topic of this review. In particular we focus on carbon fibre reinforced plastics (CFRP), as these are composites with superior mechanical properties, a high resistance to corrosion, and are considered to be important materials for the future of clean energy technology. Through this review we aim to elucidate the relevance and applicability of hygrothermal aging to the understanding of CFRP composites in marine structures such as tidal turbine blades.