The differences in the interpretation of the direct effect of a constitution are mainly caused by the peculiarities of understanding the essence of the constitution and the rule of law, the relation between the national, supranational and international law in a legal system, the specifics of the perceived constitutional concept of basic rights, the limits of private autonomy and the action of constitutional rights in public and private relations. Furthermore, a model of constitutional review and the national judicial system play a key part. The American approach, based on the distinction between common law and constitutional law, creates difficulties in the constitutionalization of the common law. These problems, along with the peculiarities of American federalism, are reflected in the state action doctrine as a tool outlining the mechanisms and limits of constitutional rights and the state’s duties to protect them. However, the solution of the question about the horizontal application of the Constitution depends, rather, not on objective criteria, but the discretion of the court. A different model of the direct action of the Constitution and basic rights has been developed in the German legal system. The Federal Constitutional Court of Germany opened the way for the constitutionalization of sectoral legislation, as well as for the recognition of the mediated horizontal action of constitutional rights in private relations. Post-socialist states developing within the continental legal family, mostly without any reservations, accepted the idea of direct action of the Constitution and fundamental rights in vertical and horizontal relations. This is largely due to the socialist tradition: the Constitution is not perceived as an act addressed to the state, but is regarded as an act addressed to the whole society. In this regard, the current Russian Constitution enshrines the universal obligation for public and private actors to observe the Constitution and allows it to be applied in private relations. The principle of the direct effect requires the court to detect possible conflicts and solve them, using available constitutional means. This mechanism of courts applying the Constitution still needs fine-tuning. In this regard, the issue of ensuring that courts apply the Constitution continues to be a challenge for Russia.