One thing is to harmonise EU company law, another thing is to ensure that the harmonised law is enforced properly. There are indications that there may be enforcement problems in this area. Such problems can be mitigated or even eliminated if EU law sets clear standards for how enforcement should be conducted. Against this backdrop, this article examines what EU law requires from the Member States when enforcing EU company law. Even though the harmonisation of EU company law seldom addresses the question of how the rules should be enforced, EU law in different ways sets bounds to how the enforcement should be conducted. These bounds are set by the requirement for effective enforcement of EU law formulated by the CJEU by free movement rights, fundamental rights and other general principles of EU law. There are so many cases dealing with this issue that it is possible to outline these bounds mainly by analysing cases dealing with company law issues. As a consequence of these bounds, the Member States may have some discretion in deciding the penalties, but they are far from free to do so. If there are major differences in how Member States enforce EU company law, this may cause major problems for the functioning of the internal market. For this reason there is a need to keep a close eye on the enforcement of EU company law in the Member State and, if necessary, harmonise aspects of the enforcement.