This book reviews recent progress in the theory of oligopoly and market leadership and provides new results on the theory of Stackelberg competition and Nash competition with strategic investment under endogenous entry. These theories are applied to models of competition in quantities, prices and to patent races. The results are used to propose a new approach to competition policy, and in particular to issues of abuse of dominance, which goes beyond the post-Chicago approach, and to analyse antitrust cases as the Microsoft case.