The content of thesis is focused primarily on emphasizing the special nature of treatment against persons with mental disorders who have committed an unlawful act in a state of insanity. Uncountable perpetrators, as a special category of offenders, are not subject to regular criminal proceedings, but to a special type of mixed proceedings characterized by two stages. Its peculiarity lies in the fact that it is divided into proceedings before the criminal court, which determines the commission of an unlawful act in a state of insanity and determines forced accommodation or psychiatric treatment at liberty, while the second part of the proceedings is carried out before the county court, which is competent to carry out the procedure of forced accommodation or psychiatric treatment at liberty. The thesis deals with changes in Croatian legislation in relation to persons with mental disorders, their correlation with international instruments of protection, the practice of the European Court of Human Rights in criminal proceedings and forced accommodation proceedings, as well as the practice of the Constitutional Court of the Republic of Croatia. Using the institute of uncountability as a starting point in the treatment of defendants with mental disorders, the fundamental determinants of countability as an ingredient of guilt, the legal nature of proceedings against uncountable offenders, together with the most striking differences compared to the treatment of defendants in regular criminal proceedings, as well as the fundamental features of the institution of forced accommodation and psychiatric treatment at liberty are discussed. For the purposes of thesis, two interviews were conducted at the criminal department of the Municipal State Attorney's Office in Zagreb and one interview with a permanent court expert, an expert in forensic psychiatry, in the premises of the "Vrapče" Psychiatry Clinic in Zagreb.