The properties of immiscible polymer blends are mainly controlled by their phase morphology. Two major classes of phase morphologies can be distinguished in a binary two-phase blend: isolated particles dispersed in a matrix and a two-phase cocontinuous morphology. The cocontinuous two-phase morphology consists of two coexisting, continuous, and interconnected phases throughout the whole blend volume. A large number of homopolymer pairs exhibit partial miscibility when mixed together either in a melt-blending process or via precipitation from a single solution. A characteristic temperature-composition phase diagram is then identified for each pair of partially miscible homopolymers. Two different main mechanisms of decomposition are known — binodal and spinodal mechanisms by which a partially miscible blend evolves to an immiscible state. Processing conditions or reactions occurring in one or both phases of the blend can strongly affect phase inversion. Based on the percolation theory, Lyngaae-Jorgensen, Utracki, and coworkers derived an equation to predict the range where cocontinuity occurs in immiscible polymer blends.