The term "alloy" as pertaining to polymers has become an increasingly popular description of composites of polymers, partiΒ cularly since the publication of the first volume in this series in 1977. Polymer alloy refers to that class of macromolecular materials which, in general, consists of combinations of chemically different polymers. The polymers involved in these combinations may be heteroΒ geneous (multiphase) or homogeneous (single phase). They may be linked together with covalent bonds between the component polymers (block copolymers, graft copolymers), linked topologically with no covalent bonds (interpenetrating polymer networks), or not linked at all except physically (polyblends). In addition, they may be linear (thermoplastic), crosslinked (thermosetting), crystalline, or amorphous, although the latter is more common. To the immense satisfaction - but not surprise - of the editors, there has been no decrease in the research and development of polymer alloys since the publication of the first volume, as evidenced by numerous publications, conferences and symposia. Continued advances in polymer technology caused by the design of new types of polymer alloys have also been noted. This technologΒ ical interest stems from the fact that these materials very often exhibit a synergism in properties achievable only by the formation of polymer alloys. The classic examples, of course, are the high impact plastics, which are either polyblends, block, or graft coΒ polymers composed of a rubbery and a glassy polymer. InterpeneΒ trating polymer networks (IPN's) of such polymers also exhibit the same, or even greater, synergism.