Primary extragonadal germ cell tumors (EGCTs) of the retroperitoneum are rarely encountered. In most cases, they arise from remnants of the genital ridge, whereas the majority of retroperitoneal germ cell tumors are metastases from primary testicular tumors. Differentiating between these primary and secondary tumors can be difficult because gonadal germ cell tumors can regress or develop slowly. Careful examination of the testes with ultrasound (US) is crucial for this determination, but asynchronous or metachronous lesions in both the testes and retroperitoneum can occur. Symptoms are often nonspecific, and diagnosis is frequently delayed. Serum markers such as beta-human chorionic gonadotropin and alpha-fetoprotein are often helpful in the diagnosis and follow-up of these tumors. Imaging techniques such as US and computed tomography play a major role in the localization and differentiation of primary and secondary retroperitoneal EGCTs. Treatment of EGCTs usually requires a combination of surgery, irradiation, and chemotherapy.