Book excerpt: Does seller concentration in a market raise prices? Many attempts have been made to test this classic hypothesis of oligopoly theory, none of them convincing. Leonard Weiss and his colleagues have devised and applied a systematic set of direct tests of the concentration price hypothesis. In an innovative series of empirical studies, they examine the effect of concentration on price for the same item sold in markets that vary because of space, time, or transaction. They conclude that concentration does indeed tend to raise price. Studies in the book's first part test specific aspects of the concentration price hypothesis. These include a case study of Portland cement deregulated fares, the relation between change in price and change in concentration in the US and in the EEC, the effect of the numbers of bidders in auctions, and the effects of concentration on wages. The book's second part brings together for the first time previously published and widely scattered studies of the concentration price relationship in advertising media, retailing, the railroads, livestock purchasing, and banking. Viewed together, they provide powerful support for the role of concentration in determining price. Leonard W. Weiss is Professor of Economics at the University of Wisconsin, Madison.P>