The horizontal effect involves the application of the ECHR in interindividual relationships. Justified by the theory of positive obligations, this jurisprudential construction allows to establish the international responsibility of the State when it fails to take necessary measures to prevent and suppress violations of the Convention committed by private persons. Recognition of the horizontal effect by the European judge results in the proliferation of substantive and procedural obligations of the State. Transposing them into domestic law compels public authorities, including courts, to interfere in private relations. Necessary for the effectiveness of human rights, the diffusion of the horizontal effect generates, in return, an extension of State intervention in the private sphere and the emergence of individual duties based on the respect of the rights and freedoms of others. The European Court of Human Rights, pioneer of the « horizontalization », refuses to develop a general theory. The challenge is therefore to delimit the extension of human rights to relations between individuals so that it does not become a factor of weakening of the vertical protection of human rights.