In economics, industrial organization is a field that builds on the theory of the firm by examining the structure of (and, therefore, the boundaries between) firms and markets. Industrial organization adds real-world complications to the perfectly competitive model, complications such as transaction costs, limited information, and barriers to entry of new firms that may be associated with imperfect competition. It analyzes determinants of firm and market organization and behavior on a continuum between competition and monopoly, including from government actions.

There are different approaches to the subject. One approach is descriptive in providing an overview of industrial organization, such as measures of competition and the size-concentration of firms in an industry. A second approach uses microeconomic models to explain internal firm organization and market strategy, which includes internal research and development along with issues of internal reorganization and renewal. A third aspect is oriented to public policy related to economic regulation, antitrust law, and, more generally, the economic governance of law in defining property rights, enforcing contracts, and providing organizational infrastructure.

The extensive use of game theory in industrial economics has led to the export of this tool to other branches of microeconomics, such as behavioral economics and corporate finance. Industrial organization has also had significant practical impacts on antitrust law and competition policy.

The development of industrial organization as a separate field owes much to Edward Chamberlin, Joan Robinson, Edward S. Mason, J. M. Clark, Joe S. Bain and Paolo Sylos Labini, among others.