This thesis examines the uses of the Amicus Curiae legal expedient by non-governmental organizations in defense of human rights, in Brazil, between 2000 and 2016. The Amicus Curiae becomes an important legal figure for legitimation in cases that aim at concentrated constitutional control, since it enables de active participation of society by way of entities, branches, groups, and institutions that express the principles and values of the majority of the population. More specifically, this thesis emphasizes that the activity of entities in the so-called Third Sector favors the legitimation of legal opinions on norms that will regulate collective interests, and that such legitimation is used as a source of juridical security in cases of (political) activism in the judiciary branch. Employing both qualitative and quantitative analysis, this thesis examines cases in which entities classed under FASFIL/IBGE s criterion 9 (development and protection of human rights) were accepted, and acted as Amicus Curiae in Cases of Direct Unconstitutionality (ADIN s) in the Brazilian Supreme Court between 2000 and 2016. Civil society, ever more institutionalized, points to a professionalization of conflicting narratives and their orientation towards hegemony, strongly anchored in the general will, as the use of the Amicus Curiae legal institution seems to reveal. Such an expedient, whose affirmation as an instrument of representation of social life is still in process, indicates the desire to occupy (as well as being invited to participate in the community of interpreters in order to assure the legitimacy of activist judicial decisions) the arena of democratic representation and participation.