Yersinioses are zoonoses inspite of the fact, that the pathogen is rarely transferred directly from the animal to man, but mostly from contaminated food or contaminated surroundings. Enteritis or enterocolitis develop in most cases after infection with Y. enterocolitica (enteritic form), whereas pseudoappendicitis accompanied by massive mesenterial lymphadenitis of the ileocecal lymph nodes with or without the participation of adjacent parts of the intestine develops after infection with Y. pseudotuberculosis (pseudoappendicitic form). Some complications like arthritis and erythema nodosum occur rather frequently and may help to establish the diagnosis; septicemia occurs very rarely and is as yet lethal in 50% of cases. Confirmation of yersiniosis is in general not possible on clinical grounds only. Also microscopic as well as macroscopic morphological findings will allow a tentative diagnosis only. Confirmation must come from bacteriologic or serologic findings.