This article discusses the study of the parties into ICSID arbitration, in the first place, investors and states as the disputing parties in arbitrations on investment and, secondly, the called Amici Curiae, who act as third parties within these procedures. The author analyzes conditions to be met by each one of these individuals in order to access into ICSID jurisdiction, and through review of existing arbitral decisions on the matter, not always coincide and uniform, identifies major trends that courts follow today.