This chapter analyzes different ways to synthesize multiphase thermosetting polymers where at least one of the phases exhibits a characteristic dimension in the range of 1 to 100 nm. Thermosetting polymers are polymer networks formed by the chemical reaction of monomers, at least one of which has a functionality higher than two. Morphologies produced by the self-assembly of block copolymers in the initial solution with thermoset precursors may be also preserved by adding another block to the original immiscible-miscible blocks. The initial miscibility of the components present in the system was identified as one of the key factors to controling the final dimensions of the generated domains. In the case of phase separation in the course of polymerization, a high initial miscibility leads to phase separation at conversions close to gelation or in the postgel stage, resulting in nanosize domains.