The Energy Charter Treaty (ECT) is a multilateral treaty in the energy sector, entered into force in April 1998, which gained worldwide recognition mainly because of its provisions on the promotion and protection of investments. The ECT currently has over 45 contracting parties. Its popularity increased significantly in the last few years following the proliferation of disputes submitted to arbitration under Article 26 of the ECT, while investors became more familiarized with the rights granted by the ECT in respect of their investments. At present, more than 20 disputes between investors and contracting parties have been submitted to arbitration. The requirements of ‘investor’ and ‘investment’ are essential for the benefit of the protection offered by the ECT. The book suggests a comprehensive approach to the notion of ‘investor’. It analyzes the notion of ‘investor’ in an inclusive manner, starting with the concept of Contracting Party to the ECT and its implications in respect of the notion of ‘investor’, going through nationality, permanent residency, dual nationality, companies and other legal entities and shareholders, to the notion of ‘investment’ and its role in defining the concept of ‘investor’ under the ECT. The aim of the book is to shed light on the proper meaning of the notion of ‘investor’ under the ECT, considering the close relationship between the ECT and other treaties such as the ICSID Convention and arbitration rules.