The central issue in this paper is the creation of an internal market for audit services within the European Union (EU). It provides an analysis of the impact of differences between national auditing regulations on intra-EU trade in audit services. To that end, the paper distinguishes three forms of intra-Union crossborder trade in audit services: (1) cross-border establishment as an auditor; (2) cross-border provision of audit services; and (3) cross-border control of an audit firm. The paper also distinguishes four categories of regulation of auditing: (I) regulation of the qualification as an auditor; (II) regulation of competition between auditors or audit firms; (III) regulation of control of audit firms; and (IV) other regulations. For each of the three forms of intra-Union trade the paper indicates the importance of each of the four categories of regulation as trade barriers. For each of the three forms of trade the current level of intra-EU trade is then discussed. The paper shows that there is little such trade and argues that given the intra-EU auditor regulation differences, audit markets within the EU are still separate markets. This is of course remarkable against the background of the drive towards more market integration in the EU in general. The paper then uses insights from audit market research to suggest which forms of national audit market regulations in EU Member States should be removed to increase intra-EU trade in audit services. The paper identifies existing national regulations of the control of audit firms as the most important barrier. The paper argues that removing these (strict) regulations is the most promising route towards more intra-EU trade in audit services.