The paper summarizes the latest knowledge of carcinoma in situ in testicular tissue. It discusses their morphological, cytogenetic and histochemical characteristic including diagnosis. It stresses the necessity of correct fixation of bioptic samples of the testes. It deals with the malignant character of the cells of carcinoma in situ and reported data supporting their invasive potencial. Further the paper shows the incidence of this finding in at-risk groups as well as clinical and pathological correlations.