A crucial factor that affects all composite materials exposed to typical service conditions is environmental degradation. It has been shown that over time the combination of moisture and fluctuating temperatures degrades the structure of polymeric composites through chemical and morphological degradation mechanisms. Experimental investigations have not only demonstrated degradation of the polymeric matrix but also of the CNF–matrix interface due to surface morphology variation caused by weathering. This chapter aims to focus on the moisture effects on the structure and mechanical properties of CNF/polymer composites in an effort to offer insight into the mechanisms of degradation and failure under load after moisture exposure. However, information on this aspect of CNF/polymer composites is in the nascent stage and extensive future work is required.