Over the past 50 years, and particularly in the last 20 years, theories of the firm have developed in diverse ways, and theoretical rivalries surrounding the modern corporation have intensified. The neoclassical model, which assumes perfectly competitive markets and profit maximization by firms, has diminished in significance, and since the 1930s, theories of imperfect competition and oligopoly, managerialism, and new institutional economics began to develop. As interest grew in corporate behavior corresponding to changes in market structure, research on corporate strategy under monopolistic competition and oligopoly has been actively conducted. In particular, with the emergence of large corporations in the late 19th and early 20th centuries, managerialist theories were proposed, whose main argument is that as ownership of large corporations became dispersed and ownership was separated from control, managers came to exercise control, and economic concentration deepened. The new institutional theory of the firm was greatly influenced by managerialism and, over the past 20 years, has demonstrated its ability to broadly explain the organization and behavior of firms from the perspectives of transaction cost economics, property rights theory, agency theory, and evolutionary economics. Theories of the modern corporation have been formed primarily around managerialism and the new institutional economics that critiques it. The two theories share the commonality of recognizing the firm as an institution, but they differ methodologically in their choice between holistic and individualistic approaches, and substantively in whether they view the firm as a social and historical institution or as the result of rational individual choice. As such, theories of corporate behavior are highly complex, diverse, and also incomplete. Nevertheless, by elucidating the implications of the firm as an actually existing economic agent, we can find clues to a desirable economic system. This paper offers new possibilities for contributing to this endeavor by providing a comprehensive overview of the theories of the firm proposed to date.