The research topic of the present work concerns the abusive tax avoidance in the field of direct taxation, while a specific reference to reorganisations of companies is made, mainly in the light of the Merger Directive 2009/133/EC regarding the implementation of a common system of taxation applicable to cross-border reorganisations of companies in the European Union. The present work, whose principal purpose is to define the concept of tax abuse, is composed of six chapters, including, among others, a comparative analysis of the anti-avoidance rules of other member states of the EU, the study of the tax avoidance measures that were adopted by the national and EU legislator to combat abusive tax practices, the issue of compatibility of anti-abuse tax provisions with the European Union law and the Tax Treaty law, as well as the relevant case law of the Court of Justice of the European Union.