Compatibility in the sense of a tendency to molecular mixture and mutual solubility is rare for two solid polymeric components. Thus, not only mixtures but also graft and block copolymers constitute two phase systems. Possibilities for identification of separate phases are discussed, including optical methods and determination of freezing and melting temperatures of the individual phases. Problems of preparation of two phase mixtures and the phase distribution arising thereby, including identification of disperse and continuous phases are discussed. Examples are given of little known polymer mixtures for which complete or partial compatibility exists.