ABSTRACTNormal nephrogenesis is reviewed in order to distinguish it from nephroblastomatosis. A number of features of the biology and behavior of Wilms' tumor show the overlapping effects of teratologic, chromosomal, and genetic influences on the genesis of this neoplasm. The age at presentation and histologic types of Wilms' tumors are also reviewed briefly in view of their relevance to nephroblastomatosis.