The European Commission (EC) discusses, in its regulation draft of November 30, 2011, the interdiction of combining audit and non-audit services by the external auditor to ensure appropriate accounting and audit quality.Based on the agency-theoretical definition of the auditor's function within corporate governance, results of empirical audit research on the effects of combining audit and non-audit services on accounting and audit quality are laid out in a state-of-the-art analysis to give a policy advice. The study at hand analyzes, whether the interdiction of combining audit and non-audit services has a negative impact on the quality of financial accounting and external audit as the EC assumes. The accounting and audit quality is measured by the variables earnings management, restatements, market reactions, and going concern opinions. So far, audit research provides an inconsistent picture, mainly in the U.S. capital market. A negative impact of an auditor offering audit and non-audit services on the quality of financial accounting and external audit can mainly only be concluded by a few variables (earnings management, market reactions). Those studies which use restatements and going concern opinions as a proxy for accounting and audit quality come to inconsistent results. Consequently, the benefit of the reform measures presently discussed by the EC is arguable. Moreover, so far, there are only a few empirical studies for the EU member states analyzing the impact of combined audit and non-audit services, thus, providing evidence for the necessity of this course of action.