The author considers the principle of reasonableness in the law, contained in all branches of the Russian and international law. Special attention is given to forms of manifestation of this principle in law. The author applies a wide variety of sources of law, assesses criteria to use the principle of reasonableness in the public and private law. Appeal to the problem allows us to extend established in theoretical science ideas about forming the principle of reasonableness as a general theoretical principle of law.

Key words: legal source, principle of law, principle of reasonableness, interpretation of law.