Analyzing the evolution process of internal audit, from its beginnings and so far, we can easily notice that internal audit function was born through detaching of some activities from external audit, the result being that there some situations when these two functions could be easily confused. The reality is that internal audit and external audit are two distinctive functions; net differenced, but which are also characterized through some complementary relationships. In this paper, we try to give our contribution at the clarification of the similarities and differences between these two functions, making a literature review that allowed us to identify some very interesting studies that permitted us to emphasize the main criterions that influenced the relationship between internal audit and external audit.