The present monograph is a synthesis of what has been contributed during the last decade to the analysis of market demand in large econ­ omies where consumers may have non-convex preference relations. Al­ though research in this field has not yet come to an end there exists a variety of interesting results, established in different frameworks by means of different conceptual and formal tools. a It is my aim to give comprehensive treatment of the existing lit­ erature including my own contributions. In working out differences and interrelations of the various ap­ proaches I adopted and modified several of the original results. My desire to present the problem and the methods by which it has been treated in such a way, that also non-specialists can follow, con­ flicted sometimes with the inevitable complexity of tools to be used. Therefore, I decided to give enough room to the introductory and prepa­ ratory part of this work. This part consists of the introduction and of the first four chapters. The main part of the present analysis consists of chapters 5 to 7.