The content of the principle of lawfulness of the restrictions of the right to liberty and security in its interpretation of the European Court of Human Rights is found out in this article. For this purpose a number of judgments in cases concerning the application of the Article 5 of the Convention for the Protection of Human Rights and Fundamental Freedoms were used. The correlation between the Conventional and national guarantees of the right to physical liberty is determined. It is drawn attention on positive obligations of the state according to the Article 5 of the Convention. It is noted that the interpretation of the Conventional concepts such as «lawfulness» and «law» can differ in some cases of the Court from their widespread understanding in the general theory of law. The formal and «essential» approaches to understanding of the concept of law in the Court case-law are compared. It is established that the term «law», used in the Court practice, covers regulations of higher legal force as well as subordinate legislation, international treaties that are the part of the sources of the national law, and well-established court jurisprudence. It is found that implementation of the restrictions on liberty in accordance with national law ctor in determining, whether such a restriction complies with the principle of lawfulness. It is explained that the lawfulness of restricting liberty implies the absence of arbitrariness in the actions of state agents, who carry out such a restriction, and its consistency with the objectives laid down in the Art. 5(1) of the Convention. Attention is paid to such attribute of law as its quality, a concept of which is used by the Court to identify the compliance of national law with obligations of the state under the Convention.