An attempt to describe known in the legal science and practice legal standards, based on the principle of reasonableness, is made in this article. The reasonableness is shown up in evaluative terms, which make the ground for the formation of standards based on the principle of reasonableness such as: «the reasonable time», «beyond the reasonable doubt» and «the reasonable grounds», «the reasonable expectations», «the obvious groundlessness». These legal standards mainly characterize the Anglo-American legal family. They are described taking into the account the practice of the European Court of Human Rights, foreign experience, national legislation and case law. In the final part it is concluded that there is a necessity to implement some legal standards into the national legal order.