In this article, the author attempts to present the sensitive issue of denunciation of Jews during the occupation. The analysis is based on Polish sources, supplemented with memoirs and testimonies. The starting point and an important intellectual context is the pioneer book by Barbara Engelking 'szanowny panie gistapo' (Dear Mr Gistapo).Informers, acting anonymously, were therefore often more dangerous than the 'szmalcownicy' (blackmailers) and did not see anything morally reprehensible in their actions. Yet it was one of the most menacing and hideous wartime occupations. The author also verifies the commonly held opinion that this phenomenon was relatively limited. It seems that this was a veritable plague during the occupation, which is reflected in literary texts. The fear of a blackmailer or informer was a feeling known to many Jews in hiding. In line with Barbara Engelking, the author treats denunciation as a form of collaboration, secret co-operation with the Nazis in their atrocious pursuits. One of the most important issues dealt with in this article is to answer why the Jews were denounced. Among the primary motives are: the desire to seize the victim's property, frustration, feelings of superiority and power afforded by the perpetrator's anonymity, but also anti-Semitism and racial hatred.