Wilms' tumor is an embryonic neoplasm of the kidney which usually presents in early childhood as an asymptomatic abdominal mass. It is the second most common solid tumor, and the fifth most common malignancy in children. It may be associated with various conditions especially sporadic aniridia, genitourinary abnormalities, and the Denys-Drash, WAGR and Beckwith-Wiedemann syndromes. The tumor has a complex genetic basis with involvement of 11p13, 11p15, 16q, 17p13 and possibly other gene loci. The multimodal approach with surgery, chemotherapy and radiation achieves excellent survival. Although relatively rare, Wilms' tumor has been the subject of intensive research as a model for kidney development, cancer treatment and molecular genetics.