A review is presented of the synthesis, morphology and thermal and mechanical properties of interpenetrating polymer networks and related pseudo-interpenetrating polymer networks in which only one of the polymer constituents is cross-linked. That the degree of phase separation is strongly dependent on the degree of compatibility of blends of the linear polymer constituents is illustrated by detailed comparisons of two classes of full and pseudo-interpenetrating polymer networks. The first consists of a polyurethane/acrylic copolymer, the components of which are relatively incompatible, and the second of a polyphenylene oxide/polystyrene, the components forming compatible blends over essentially the whole composition range. -- AATA