Inclusion of non-signatories to an arbitral agreement represents nowadays one of the most important discussions on international commercial arbitration. Treated in a great variety of theories, both doctrine and jurisprudence have tried to shape a series of concepts to enlighten the way in which this problem should be addressed. To the day, no undisputed solution has been found. The Alter Ego Doctrine is one of those vehicles under which a subject, that at first glance would not appear to be a party of the arbitral agreement, is incorporated into the dispute. Despite its continues appearance on arbitral awards and court decisions, and despite being one of the most relevant institutions in international commercial arbitration, there is still no real consensus on its essential elements and what legitimizes its use. It is probable that the answer is to be found far beyond the restrictive concepts of absolute control and fraud that gave birth to the Alter Ego Doctrine. It may be even necessary to reinterpret the inclusion-consent relation in order to understand which is the real aspect that justifies its application. As a matter of fact, the discussion is constantly growing towards a more flexible comprehension of the elements of the Alter Ego Doctrine, including the concept of a ‘close relationship’ between the companies upon which an Alter Ego is disputed.