Germ cell tumours form an important group of gonadal neoplasms and are also found in a number of extragonadal sites like the mediastinum, para-pineal and sacrococcygeal regions and retroperitoneum. Although there are considerable differences between germ cell tumours occurring in different anatomical locations they exhibit a remarkable homology, and are considered as a group. In this review germ cell tumours are discussed as a group emphasizing some of the recent developments in this field. In the testis germ cell tumours form the most common group of neoplasms comprising 90% of all testicular tumours and 99% of them are malignant. In the ovary germ cell tumours comprise approximately 20% of ovarian neoplasms, and more than 90% are mature cystic teratomas and are benign. Malignant testicular neoplasms are 10 times more common than their ovarian and 20 times more common than their extragonadal counterparts. Malignant germ cell tumours have a specific age incidence and occur mainly in children and young adults. Due to this they represent one of the most important groups of neoplasms in this age group. Testicular germ cell tumours show marked racial and geographical differences occurring much more frequently in Western Europe, especially in Scandinavia, as compared with Southern and Eastern Europe. They are rare in Africa and are very uncommon in Blacks as compared to Whites. These remarkable differences are not observed in ovarian or extragonadal germ cell tumours. It is now accepted that histogenetically all the tumours in this group are of germ cell origin, and that germ cell tumours are capable of somatic (embryonal) and extra-embryonal differentiation (fig. 1). The occurrence of extragonadal germ cell tumours in anatomical locations in the midline of the body is explained on the basis of migration of the primitive germ cells during embryonic life from the wall of the yolk sac to the primitive gonad. An all embracing classification of germ cell neoplasms based on the WHO classifications of ovarian and testicular tumours is presented. The importance of careful and thorough examination of germ cell tumours is emphasized, especially in view of the recent advances in the therapy of malignant germ cell neoplasms. The value of tumour markers like alphafoetoprotein (AFP) and human chorionic gonadotropin (HCG) produced by endodermal sinus tumour (EST) and some embryonal carcinomas and choriocarcinoma and syncytiotrophoblastic giant cells respectively in diagnosis, monitoring the progress of the disease, and the efficacy of therapy, as well, as for early detection of metastases and recurrences, is strongly emphasized.(ABSTRACT TRUNCATED AT 400 WORDS)