Professionals working in the field of psychiatry have recently been increasingly encountering patients seeking help who define themselves as transsexual. Gender identity and gender self-expression of transsexual people do not match their biological sex, and they also feel an intense desire and urge to change their officially registered gender at birth and their physical appearance - even by using hormone therapy or surgery. In their case, the diagnosis of gender identity disorder as a disease category enables the use of medical interventions and health services necessary for transition. At the same time, the diagnosis is complicated by the fact that there is currently no specific psychodiagnostic procedure suitable for the clear identification of gender identity disorder, and gender incongruence can also appear as a symptom of many other mental disorders. Therefore, careful differential diagnosis is essential to establish a well-founded diagnosis, with the help of this literature review that summarizes other mental illnesses with similar symptoms as well as the more common comorbid pathologies.