Internal audit, as a relatively new scientific discipline has an increasingly important role and is constantly on the rise. In addition to the audit of the financial statements, internal audit represents a significant segment of the overall audit activity. Over time, internal audit has developed into an infrastructure function, whose scope of work is intensifying and expanding - from financial information assessment to verification and assessment of the efficiency and effectiveness of the entire business process. The development of modern internal audit is a result of turbulent and complex business conditions and the response to the problems that arise in the traditional decision-making approach. Consequently, the internal audit must necessarily be in the context of the concept of corporate governance. This paper deals with the concepts, the basic determinants and the development of internal audit, and the detailed internal audit phases - properly implemented - should provide the quality of internal auditor activity