The aim of this paper is to analyse the institute of the amicus curiae, in the light of doctrine and jurisprudence, seeking to study its origin, definitions, procedural possibilities and the effects of its intervention, as well as to analyse the social and legal importance of its intervention in constitutional jurisdiction today. A brief history of amici curiae and constitutionality control in Brazil is presented, the relevant legislation and case law are examined, and the purpose and acceptance of this institute are discussed. The line of thought accepted by the best doctrine for tackling the issue raised is pointed out.