This Study was commissioned by the Internal Market Directorate General of the European Commission. The objective of the Study should be seen in the context of Article 26 of the Statutory Audit Directive (Directive 2006/43/EC). This provision allows the European Commission to adopt International Standards on Auditing (ISAs) issued by the International Auditing and Assurance Standards Board (IAASB) through a binding legal instrument. In line with Article 26, ISAs should be adopted only if, among other conditions, they contribute to a high level of credibility and quality to audited financial statements. The Study evaluates whether the adoption of the ISAs by the European Union (EU) would achieve this condition and analyses the related costs and benefits of such an adoption. This analysis covers the costs and benefits arising at the audit engagement, audit firm and audit client levels, as well as those arising for audit regulators, audit markets, and capital markets.