Nephrogenic rests (NR) are abnormally persistent clusters of embryonal cells, representing microscopic malformations (dysplasias) of the developing kidney. Though NR are best known as precursors of Wilms tumor (WT), many alternative fates are observed, and most rests are destined for eventual atresia. Biological and clinical distinctions between the two major NR categories, perilobar and intralobar rests (PLNR and ILNR) are emphasized. PLNR occur in fetal overgrowth and with certain overgrowth syndromes. ILNR are frequently associated with deletions or mutations of WT1. Data are presented concerning the prevalence of NR in general pediatric autopsy populations, and in selected syndromes. The age at diagnosis of WT was determined for the largest series of patients with WT-associated syndromes reported to date. These data provide a basis for determining how long patients with these conditions are at risk for WT development.