Most of the retroperitoneal tumors (not originating from organs) are extremely rare (0.1% of all tumors) and are malignant. Clinical signs occur only late, are non-specific and are related to extreme tumor size. The most frequent tumors are malignant fibrous histiocytoma, liposarcoma, leiomyosarcoma, rhabdomyosarcoma, hemangiopericytoma and neurogenic tumors. The relevant clinical and morphological findings of these tumors as well as prognostic features are discussed.