The present two volumes contain the essays and part of the discussions as presented at the conference on Mainstreams in Industrial Organiza tion, held at the University of Amsterdam, 21-23 August 1985. The thema was chosen because the field of studies commonly designated "industrial organization" in the Anglo-Saxon countries, or "market theory" in Continental Europe, has experienced important alterations during the past decade. Partly this reflects changing theoretical views inside the field, in which shifts in the core concepts have occurred and different emphasis is laid on time-honoured views and results. Partly, critical views have been voiced from outside the field. As in all open scientific debate, they have to be weighed and, if necessary, taken into account. Partly also, diver gent developments in thinking between the Anglo-Saxon, European and Japanese areas need to be considered, because both the problems and the ways of approaching them still differ. The variety of views, theori~s and results is testimony to the vitality of this field of economics; variety is generated by the creative endeavours, from which the chaff is being beaten out by critical discussions. That is especially true for the concept of competition itself, which industrial organization economists are debating intensively.