There is no internal audit theory; so far, only theories concerning the control of the activities of private sector entities are being developed. The authors have made an attempt to locate internal auditing in selected economic theories. It consisted of indicating how the internal audit contributes to solving problems arising in the organization, depending on the goals of its stakeholders. The article discusses auditing in the context of agency theory because it is recognized that the principal-agent relationship occurs when one party (the auditor) is obliged to act on behalf and in the interest of the other (the organization manager). The knowledge of potential conflicts that may arise between them may contribute to lowering the costs of their elimination. Moreover, internal audit can be a tool for solving problems resulting from the lack of (asymmetry) information. Internal auditing is also presented in light of behavioral theory, assuming that both the auditor and the auditee are guided by different premises and their behavior is influenced by various factors. Knowledge of these motives and factors is just as important in the auditor’s work as the specialist (technical) knowledge he uses in the audit process.