This master thesis investigates the influence of audit market concentration (Big 4) on audit quality on the basic of a literature review. In the existing literature, some authors pointed out that the Big 4 networks could be ?too big to fail?, which would imply a reduced audit quality. For example, in 2002 was the Enron-scandal and hence the General Accounting Office (GAO) enacted the Sarbanes-Oxley-Act (SOA). In the empirical papers, the measurements for the market concentration and audit quality is based on different types. The market concentration is measured by the share of the Big 4 audits (as a group) and with the Herfindahlindex (distribution within the Big 4). The measurement for the audit quality date from Linda Elizabeth?s quasi-rent approach and different models of discretionary accruals are used as proxy-measures. The main paper from Francis et al. (2011) has by far the largest sample size and the results reveal that a high proportion of Big 4 audits are positively related with all measurement of the audit quality of all auditors. In contrast to this, an unequal distribution of the audits within the Big 4 leads only among the Big 4 to a lower audit quality. Not all studies are in agreement with these results and further studies show that not only the national but also the local aspect of market concentration has a significant impact on audit quality. According to this, greater local auditors imply a higher audit quality. The results to explain the relationship between market concentration and audit quality are contrary, because further studies restrict or negate the above results. For example ? relating to Iran and excluding AO, Dehkordi/Makarem (2011) evidence no significant differences in audit quality between the non-Big 3 and Big 3 there and for all types of auditor changes. The papers relating to Korea and Belgium confirm qualitatively the results from Iran. A possible explanation is the specific situation of each country.