The article is dedicated to the role of the agency theory in the development of an interdisciplinary theory of the firm. The agency theory is presented as a model that may incorporate findings from other disciplines. An expanded agency theory could contribute to a holistic theory of the firm, yet it is uncertain if such a theory could be developed at all. The first part of the article offers a comprehensive description of the agency theory, its assumptions and conclusions. The author analyzes the relationship between a principal and an agent of the principal. He also describes the sources of asymmetry of information-in the form of moral hazard and adverse selection-along with a presentation of monitoring strategies and incentives used by principals. Finally, the limitations of the agency theory are discussed. The second part of the paper deals with the challenge of expanding the agency theory to include a theory of power in the firm. This approach is applied to analyze power relations in the agency relationship. The sources of the principal’s and the agent’s powers are traced in terms of their development over time. The role of ideology in the agency relationship is also considered. The article concludes with a summary of the results of combining the agency theory with the theory of power.