Multicomponent polymeric systems, or polymeric blends, have recently created considerable interest and they represent a new and important challenge for research. These systems have already become technologically important, but the prospects for their applications have by no means been exhausted. For thermodynamic reasons polymer blends do not usually form homogeneous mixtures but exhibit micro or macrophase separation. This incompatibility has some inherent advantages as varying the composition and the processing conditions, materials with different structures and morphologies can be obtained whose properties may be superior to those of one of the components e. g. high impact resistant plastics. Investigations of multicomponent polymer systems constitute a new branch of macromolecular science which now claims as much interest as the behaviour of dilute solutions, crystallization, statistics of chain~~ tacticity, and single crystal formation did a few years ago. The complexity of the problems related to control led preparation and properties studies of mUlticomponent polymer systems is such that it is often more practicable to conduct them on an international basis. The aim of the first Joint Italian-Polish Seminar on Multicomponent Polymer Systems was to discuss recent results obtained in that field in both countries. As the range of topics to be covered was fairly wide, we thought it advisable to invite scientists from other countries to share with us some aspects of their own studies.