The peculiarities of reaction-induced phase separation and the structure formation in semi-and full interpenetrating polymer networks and in the blends of linear polymers formed in situ are analyzed. It is shown that for most of these systems phase separation proceeds via the spinodal decomposition mechanism resulting in the formation of interconnected spatially periodic structures. The possible ways for the structure regulation of the composites produced are considered. (C) Koninklijke Brill NV, Leiden, 2011