Over the last ten years, it has been possible to observe the development of a growing phenomenon, the expulsion of European Union citizens from their host Member State. While Union citizens are encouraged by the Union to use their fundamental right of the freedom of movement, Union citizens moving to other Member States still encounter legal obstacles, possibly leading to their expulsion. Recently, some Member States even demonstrated a strong political will to reconsider the benefits of the principle of free movement, which has been built progressively since the foundation of the European Community. This restrictive approach is taking place in the context of the global economic crisis, which occurred just after the enlargement of the EU to economically poorer countries from Central and Eastern Europe, leading to more nationalistic and protective measures, which have legal consequences on mobile Union citizens. This paper intends to analyze the legal grounds for expulsion of Union citizens issued by EU law such as threats to public policy, public security or public health, abuse of law, fraud and unreasonable burden to the national social security system. Special attention will be given to the national interpretations of these legal institutes which are too generally defined in the Citizenship directive. The paper will use a comparative approach.