The pathogenesis of pediatric malignant tumors is associated with congenital abnormalities. Oncogenes and antioncogenes are identified in some of these cases. Neuroblastoma arises from the adrenal medulla and sympathetic ganglia. Most neuroblastomas produce catecholamine. Urinary vanillylmandelic acid(VMA)and homovanillic acid(HVA), metabolites of catecholamine, are sensitive tumor markers. Risk stratification according to tumor stage and a combination of prognostic factors helps determine the appropriate therapeutic strategy in clinical settings. Nephroblastoma(Wilms tumor)is the most common pediatric renal tumor and is often accompanied by congenital anomalies. Surgical resection of the tumor and the involved kidney is the initial treatment recommendation in the US and Japan. Consecutive chemotherapy and radiotherapy are administered after surgical staging and a definite histopathological diagnosis. Prognosis is relatively good for most nephroblastoma cases with a favorable histology. In addition to nephroblastoma, clear cell sarcoma of the kidney, characterized by a tendency to metastasize to the bone, is a renal tumor with poor prognosis. Rhabdoid tumor of the kidney is another tumor type; however, its pathogenesis is still unknown and it is associated with extremely poor prognosis because of the lack of effective therapeutic measures. Hepatoblastoma is the most common malignant liver tumor. The serum alpha-fetoprotein level is the most effective tumor marker. Complete surgical resection of the involved liver lobe is the definitive approach for cure. Preoperative chemotherapy increases the possibility of complete surgical resection. High-risk patients have a poor prognosis.