This book examines one of the classical issues in theoretical welfare economics, the effects on social welfare of increasing competition between firms. The author explores whether promoting competition is in fact desirable--an issue which is central to modern debates about the role of markets. The desirability of competition, an idea which can be traced back to Adam Smith, is widely accepted, resulting in the widespread belief that by increasing interfirm competition we may always improve social welfare. However, this is challenged by another piece of conventional wisdom which claims that excessive competition is as harmful as insufficient competition. The work will be of interest to researchers and practitioners working in industrial organization and welfare economics.