This papir will analyze legal position and the procedure of insane perpatrators. The attitude of society to them in moral and legal terms is ambivalent – society recognizes their difficult positions and need for protection, but also recognizes their danger to others. At the beginning of the modern century, insane perpatrators have been recognized as a different group in terms of the crimes that they commit and their punishments. The criminal law positions of people who may lack of mental capacity in the Republic of Croatia is regulated in the Criminal code and the Criminal procedure act, but legislation of the Republic of Croatia has changed several times, and the most important changes were in 1997. when insane perpatrators were excluded from criminal law or punishment and went beyond the scope of the Law on the Protection of Persons with Mental Disabilities. The paper deals with the specialities of expert's work in the criminal procedure, when they are used as evidence when the court does not have a certain professional necessary knowledge to know certain facts. Also the law describes implementation of security measures of involuntary admission and treatment as well as treatmant outside the hospital.