The cuticle of nematodes is a thin, flexible, outer covering composed primarily of protein with trace amounts of lipid and carbohydrate. There has been considerable recent interest in the biochemistry, immunology and molecular biology of the cticle of parasitic nematodes because of its role as an interface between parasite and host. The cuticle consists of (1) collagen-like proteins that form the medial and basal layers, (2) non-collagen proteins that form the epicuticular and external cortical regions and, (3) non-structural proteins associated with the external surface. The collagen-like proteins are solubilized by reducing agents, exhibit stage and species variations and have molecular weights of 30 to 120 kDa. Nematode collagen genes studied thus far however code only for proteins with molecular weights of 30 kDa. The noncollagenous proteins, referred to collectively as cuticlin, exhibit unusual chemical properties as indicated by their resistance to solubilization even under strongly denaturing conditions. Recent studies on Ascaris suum have demonstrated the presence of tyrosine-derived corss-links that may form the linkage between subunits in assemblage of the collagenous and noncollagenous structural components of the cuticle. A peroxidase enzyme has been implicated in the synthesis of these cross-links. Recent ¹²⁵I labeling studies of Haemonchus contortus have identified and characterized stage-specific proteins on the cuticular surface.