This article describes some of the recent developments in the pathology of germ cell tumors of the testis. Many germ cell tumors show different types of differentiation. Two different explanations for this phenomenon include the differentiation of other germ cell elements from totipotential embryonal carcinoma cells or the direct differentiation of neoplasms from a malignant intratubular germ cell. Although the concept that there is a subset of seminomas having a poorer prognosis still exists, the histologic identification of such "anaplastic seminoma" remains an unachieved goal, and we, therefore, do not recommend the use of the term anaplastic seminoma at present. A recent analysis of spermatocytic seminomas has failed to demonstrate that they are capable of meiotic division. They are composed of cells differentiating in the direction of spermatocytes, but they have not achieved that stage. The prognosis, in general, remains excellent, although recently sarcomas have been reported in association with spermatocytic seminomas with metastasis of the sarcomatous elements. The presence of human chorionic gonadotropin-producing syncytiotrophoblastic giant cells in otherwise pure seminomas does not appear to adversely affect the prognosis. Yolk sac tumors have a varied histology that many pathologists do not recognize. The presence of intercellular basement membrane (parietal differentiation) is useful in the recognition of yolk sac tumor. Sometimes solid foci of yolk sac tumor may be mistaken for seminoma, and alpha-fetoprotein and cytokeratin stains may be useful in this situation, although the presence of basement membrane, hyaline globules, and focal microcysts by light microscopy may obviate the need to use them. Hepatic and enteric (or endometrioid) differentiation may occur in yolk sac tumors and cause diagnostic confusion. The development most "non-germ" cell malignancies in patients with germ cell tumors appears to occur by transformation of aneuploid teratomatous elements at the primary or metastatic site. The identification of such malignancies depends on the recognition of invasion by the elements rather than on high-grade cytologic atypia. Unusual patterns of choriocarcinoma and yolk sac tumor may be encountered following chemotherapy, and there is circumstantial evidence that some sarcomas and carcinomas occurring in patients with testis cancer may develop directly from yolk sac tumor.