Germ Cell Tumors (GCT) represent an important group of mediastinal tumors. Because of genetic characteristics and behavior have been divided in prepubertal and postpubertal tumors, and their origin remains controversial. The genetic changes are diverse, but the most frequent is i (12p), and can be associated to gonadal disgenesis. Histological classification is similiar to that of gonadal tumors and all histological types have been described in the mediastinum. Teratomas may undergo malignant transformation with the emergence of somatic tumors such as haematological malignancies, sarcomas, carcinomas and neurogenic tumors, and such transformation may occur in the primary site or in the metastases. Prognostic factors in mediastinal germ cell tumors include: age of the patient, histological type, distant metastases, stage, status of resection, level of serum tumor markers and proliferation markers. Immunohistochemistry is especially useful when the primary GCT is occult, to separate types of immature GCT, and to determine the lineage of malignant transformation which may be important to design treatment strategies.