In order to effectively deal with the problem of so-called dangerous prisoners, the law must properly identify the conditions of isolation and particularly criteria based on which the convicts and detainees shall be qualified for this category. The purpose of this article is to present and analyze these qualification rules in the Polish penitentiary law, taking into account the content of judgments of the European Court of Human Rights in cases Piechowicz v. Poland (no. 20071/07) and Horych v. Poland (no. 13621/08) of 17 April 2012 and the planned legislative changes and their progress.