The legal status of current and future European investment agreements remains uncertain. International law and European Union law sometimes create conflicting obligations for EU Member States with respect to their existing investment treaties. Although the Lisbon Treaty gave the EU exclusive competence over foreign direct investment in December 2009, debate continues about whether the EU’s investment competences are sufficiently comprehensive that the EU could create a common investment policy. This paper traces the roots of these legal issues with European investment agreements from the pre-Lisbon era to the present day. The EU Commission proposed a regulation for transition arrangements that would "grandfather in" existing investment agreements and authorise Member States to continue negotiating such agreements. The paper describes the legal evolution of the proposed regulation, which was enacted subsequent to the writing of this paper, and analyzes the legal issues not addressed by the regulation. This paper was written to fulfill the dissertation requirement for the LL.M. in International Business Law at Queen Mary College, University of London.