Gonadal sex cord-stromal tumors contain some of the most morphologically interesting neoplasms of the gonads and these lead to many important issues in differential diagnosis. The pathology of these tumors is reviewed with emphasis on new information, similarities and differences in the two gonads, and diagnostic problems. Sertoli cell tumors occur in both gonads being more common in the testis where they usually exhibit a lobular pattern of hollow or solid tubules. In the ovary, tubular differentiation is usually the predominant feature but the lobulation typically seen in the testis is generally not as striking. One variant of Sertoli cell tumor, the large cell calcifying form, appears to be restricted to the male gonad and in contrast to other sex cord tumors is much more frequently bilateral and is associated in many cases with unusual clinical manifestations. In both sexes, patients with Peutz-Jeghers syndrome often have distinctive gonadal pathology. In females, it is in the form of the sex cord with annular tubules whereas in males, the lesion has features that are often intermediate between those of a sex cord tumor with annular tubules and a large cell calcifying Sertoli cell tumor. Sertoli-Leydig cell tumors are more morphologically diverse than pure Sertoli cell tumors and for practical purposes are an issue only in ovarian pathology being exceptionally rare in the testis. The classification proposed by Meyer into well, intermediate, and poor differentiation, remains important prognostically. More recently, heterologous and retiform differentiation has been described. Heterologous tumors most often contain mucinous epithelium, sometimes with small foci of carcinoid or less commonly, and generally in poorly differentiated neoplasms, rhabdomyosarcoma or fetal-type cartilage. Such tumors should be distinguished from pure sarcomas and teratomas. The retiform neoplasms, which tend to occur in young females, may mimic serous borderline tumors or even serous carcinomas. Granulosa cell tumors are much more common in females and in both gonads are divided into adult and juvenile forms. In females, granulosa cell tumors and other sex cord tumors may have markedly bizarre nuclei potentially leading to overdiagnosis as more malignant neoplasms. The juvenile granulosa cell tumor of the testis tends to occur in the first 6 months of life and should be carefully distinguished from the yolk sac tumor of the testis, which usually occurs in a slightly older age group. Occasional sex cord-stromal tumors cannot be readily categorized into the Sertoli or granulosa families and are diagnosed as sex cord-stromal tumors unclassified. In females, this is a relatively common placement for a neoplasm in a pregnant patient. Unclassified tumors are overall more common in males and may entrap residual normal germ cells potentially leading to the erroneous placement of the tumor in the category of a mixed germ cell sex cord-stromal tumor. From the practical viewpoint, the most helpful immunohistochemical findings are the negative staining of sex cord tumors for epithelial membrane antigen, and positive staining for inhibin and calretinin, findings that are converse to those seen in endometrioid carcinomas of the ovary, which commonly have formations that simulate sex cord tumors.