A theoretical and empirical study of the effects of competition across a broad range of industries. Policies to promote competition are high on the political agenda worldwide. But in a constantly changing marketplace, the effects of more intense competition on firm conduct, market structure, and industry performance are often hard to distinguish. This study combines game-theoretic models with empirical evidence from a "natural experiment" of policy reform. The introduction in the United Kingdom of the 1956 Restrictive Trade Practices Act led to the registration and subsequent abolition of explicit restrictive agreements between firms and the intensification of price competition across a range of manufacturing industries. An equally large number of industries were not affected by the legislation. Using data from before and after the 1956 act, this book compares the two groups of industries to determine the effect of price competition on concentration, firm and plant numbers, profitability, advertising intensity, and innovation. The book avoids two problems common to empirical studies of competition: how to measure the intensity of competition and how to unravel the links between competition and other variables. Because the change in the intensity of competition had an external cause, there is no need to measure the intensity of competition directly, and it is possible to identify one-way causal effects when estimating the impact of competition. The book also examines issues such as the industries in which collusion is more likely to occur; the effect of cartels and cartel laws on market structure and profitability; the links between competition, advertising, and innovation; and the constraints on the exercise of merger and antitrust policies.