Amici curiae are persons interested in a trial but not party to it that submit an unsolicited written brief or make an oral statement before the bench. The widespread possibility to submit amicus curiae briefs in international courts and tribunals is a recent phenomenon. Traditionally international procedures did not allow this kind of intervention. The purpose of this paper is to take stock of this evolution and assess the functioning of the new procedures. It does this by looking comparatively at several courts and tribunals, in order to get a picture of the commonalities and common problems surrounding this general development. Two sets of questions are considered, the first centered on issues of transparency and public participation, the second on the rights of the parties. Among the elements that deal with public participation, the paper examines the clarity of the procedures, the equality of the treatment of all the interested entities, the conditions and reasons for accepting or refusing the proposed amici, and the inclusion of the amici submissions in the text of the final decision. The other set of questions, concerning the rights and interests of the parties to the dispute, includes their role in the submission phase and their interest in efficient proceedings. In answering these questions, serious issues of transparency, publicity, and the political role of states acting as amici emerge. The paper concludes that further reflection is necessary on the origins of these problems, but that a first, important step can be achieved by addressing these procedural issues.