The present study provides an overview of the pertinent legal issues involved with a mandatory EU lobby register, discussing the legal basis, form and potential contents of such a register. The study assesses if there is a legal basis in current EU primary law to establish a mandatory lobby register. The question of the legal basis is of special interest, because the apparent lack of such a legal basis is a common argument against a mandatory register. In particular, the opposition of the European Commission against a binding register seems to be partly based on the assumption that the current treaties do not contain a sufficient legal basis for a register. The study will therefore also discuss how such a legal basis could be established if the treaties in their current form are not deemed to be sufficient. In addition, the study also addresses other possibilities to make the current regime more effective through stricter staff rules or codes of conducts of the European Commission or Parliament.