The potential conflict between domestic labour law and free movement provisions set out in the European Community (EC) Treaty has long been the subject of academic and political comment.1 After much speculation, we now have some legal guidance on this matter, although it may not be sufficient to allay entirely any concerns.2 Notably, the guidance given does not take the form of EC legislation or an amendment of the EC Treaty. It has not been achieved through political agreement between the Member States, for it would seem that the differences between the EU-15 States and those who joined since enlargement render such agreement highly unlikely.3 Rather, the European Court of Justice (ECJ) has, through the application and further development of the principles established in free movement cases and in its general principles jurisprudence, indicated how exercise by trade unions of a right to strike may be restricted in the light of an employer’s entitlement to freedom of establishment and freedom to provide services. The judgment, delivered in International Transport Workers’ Federation (ITF) and Finnish Seamen’s Union (FSU) v. Viking Line,4 is likely to have significant consequences for trade unions which seek to resist re-flagging of vessels within the European Union.