The design of the external inspections and quality control of auditing process contributes to the credibility of statutory annual audits. The external inspection can be basically conducted either in form of self-administration or take place under a regulatory oversight. Although the self-administration is characterized by a considerable level of flexibility, the independence and credibility of the external oversight has been recently questioned. As a reaction to the European audit reform the German legislator has changed the national auditing regulation in December 2015 and implemented an independent state supervision authority, which especially oversees the auditing process of public interest entities (PIEs). Auditors entrusted with the assurance of PIEs are subject to the supervision by the newly implemented authority (so called Abschlussprüferaufsichtsstelle – APAS) organized under the German Federal Office for Economic Affairs and Export Control. One of the main issues under discussion is the spillover effect of this regulatory reform on the external oversight of the assurance of non-PIEs. Based upon the agency theory this article investigates, how this European regulatory reform has been transformed into the national auditing regulation in Germany and includes the results of the empirical audit research for the USamerican Public Company Accounting Oversight Board (PCAOB). Under special consideration of commensurability several selected aspects of the auditor oversight regulation reform will be discussed.