Jealousy is defined as a complex mental state caused by a real or false – imaginary – change in a way of perception of a threat of terminating a valued partnership in favor of a real or imaginary rival. Apart from jealousy demonstrated by mentally disordered people, but without symptoms of mental illness and obsessive jealousy, there is also a psychotic jealousy. One of diagnostic difficulties is to determine when a normal jealousy about a partner acquires delusional nature and the boundary between these two can be difficult to detect, which often is a concern of forensic psychiatrists. A mistake in diagnosis can seriously affect issuing of opinions and can have legal consequences. There are accounts claiming that the emergence of delusional jealousy is a continuous process, from the so-called normal to morbid jealousy. Diagnostic assessments in this matter bring difficulties in juidicial-psychiatric assessments. Delusions of marriage infidelity accompany various psychotic disorders and their foundations can be found in endogenous factors, organic changes in peripheral nervous system and addictions. People demonstrating these types of delusional disorders are extremely dangerous for the environment, not solely because of the presence of psychotic symptoms, but also because of various environmental and relationship-related factors, which contribute to the genesis of their criminal offenses. Making accurate and just forensic psychiatric assessment of this group of offenders concerns risk assessment of offenders, validity of the use of security measures and the type of therapeutic interventions.