The European Court of Justice (ECJ) has issued a long-anticipated judgment on the interpretation of who is the ‘employer’ of a cross border worker. This determination is important in European social security coordination legislation (Regulation no. 883/2004 and Regulation no. 987/2009) because where the employer is registered may determine whether social security will be payable in one member state or another. In this recent case, the court ruled that the employer was the undertaking that in reality exercised the functions of the employer rather than the other undertaking with which the employees had their formal employment contracts.