Since at least 20 years, most of the legal proceedings of marital incapacity are carried on by the church tribunals mainly of no3 canon 1095 new law code, that is of mental incapacity to undertaking and performing the essential marital duties. Very often the legal proceedings are very difficult and the canonical lawyers need the forensic psychiatric or psychological opinions. The aim of this paper is to give the forensic psychiatrists and psychologists the essential knowledge connected with the new regulations listed in canon 1095 no 3 of the actual canon law code. According to the opinions of the notable canonical lawyers the author describes the legal and marital prerequisites for valid marriage, the ability for undertaking and performing the essential marital duties and the mental reasons for marital incapacity. The author points out several psychiatric reasons underlying the mental incapacity for undertaking and performing essential marital duties: personality disorders and alcoholism which preclude adequate interpersonal relations and sexually related personality disorders which preclude the exclusiveness and natural consumption of the marital agreement. At the end the author points out the conditions which should have good forensic opinion.