How do economists view the overall effectiveness of antitrust policy?

Most economists conclude that, overall, U.S. antitrust policy has been moderately effective in achieving its goal of promoting competition and efficiency. Much of the success of antitrust policy arises from its deterrent effect on price-fixing and anticompetitive mergers. Antitrust policy has not been effective at restricting the rise of or breaking up monopolies but has been somewhat effective against abusive monopoly behavior. This effectiveness is diminished by a slow legal process.