Book excerpt: This timely work seeks to identify the differences between the domestic procedural rules and principles of an array of EU and non-EU countries and analyse them in the context of European Union law requirements. Specific attention is paid to the impact of State aid rules on procedural law in tax matters, on constitutional law requirements as well as tax treaty law issues. Since customs law is already harmonized in the form of the Community Customs Code, it serves as a starting point to examine the extent to which harmonized procedural law is possible. Harmonized procedural law is also discussed in the context of a possible future Common Consolidated Corporate Tax Base as well as an EU tax levied at the European Union level.