THE Enforcement of antitrust laws has been the responsibility of the Antitrust Division of the Department of Justice and the Federal Trade Commission (FTC). Antitrust violations have been resolved by dissolution and divestiture, injunction, and consent decree [see Problem Fines and jail sentences have also been imposed. Starting with the 1945 Alcoa case, the U.S. Supreme Court ruled that size per se is an offense, irrespective of illegal acts. Today, however, both size and some anticompetitive behavior seem to be required for successful prosecution. The Court has generally challenged horizontal mergers between large direct competitors, but has not challenged vertical and conglomerate mergers unless they would lead to increased horizontal market power. The Court has used the Sherman Act to prosecute not only attempts to set up a cartel, but also any informal collusion to share the market, fix prices, or establish price leadership schemes. The Court has ruled that conscious parallelism (i.e., the adoption of similar policies by oligopolists in view of their recognized interdependence) is illegal when it reflects collusion. The Court has also attacked predatory pricing (i.e., selling at below average variable cost in a particular market in order to drive a competitor out or to discourage new entrants) and price discrimination and other price behavior when it substantially lessens competition or tends to create a monopoly. Since the mid-1970s, the government has deregulated airlines and trucking, and reduced the level of regulation for financial institutions, telecommu¬nications, and railroads in order to increase competition and avoid some of the heavy compliance costs of regulation. While the full impact of deregulation has yet to be felt, deregulation seems to have led to increased competition and lower prices, but it has also resulted in some problems.