In this paper, we review the synthesis, morphology, and physical and mechanical properties of IPN’s as well as the related pseudo-IPN’s in which only one of the polymers is cross-linked. Recent studies have shown that the degree of phase separation achieved in these materials is strongly dependent on the compatibility of blends of the linear polymer constituents of the IPN components, as well as the kinetics of chain extension and the presence of grafting between component polymers. We illustrate this by a series of IPN’s consisting of a polyurethane and an acrylic copolymer. The acrylic is a typical automotive enamel. An enhancement in properties results which is dependent on the amount of grafting and the kinetics of polymerization. Also discussed are IPN’s of a polyurethane and an epoxy, which exhibited a synergism in adhesive properties, and IPN’s of a RIM polyurethane with several epoxies and unsaturated polyesters.