The main recommendation of this paper is that private international company law in the EU should be harmonised. The paper discusses the main challenges that a future regulation to this effect – called here ‘Rome V Regulation on the Law Applicable to Companies’ – would have to overcome. Some of those are of a political nature: for instance, countries may fear that it may become easier for companies to evade domestic company law (eg, rules of employee co-determination), and there are specific considerations that concern companies established in third countries. Another challenge is that a future regulation on the law applicable to companies has to be consistent with existing EU conflict of laws rules as regards, for example, insolvency and tort law, while also complying with the freedom of establishment of the Treaty. It is the aim of this paper to discuss these questions in detail, notably the general considerations for harmonisation in this field, a potential harmonisation based on the ‘incorporation theory’, how it may be possible to overcome some contentious issues such as the definition of the lex societatis or the relationship between the lex societatis and other areas of law, and the prospects of future international harmonisation.