Wilms' tumour (WT; nephroblastoma), a kidney neoplasm, is one of the most frequently occurring solid tumours of childhood. It arises from the developing kidney by genetic and epigenetic changes that lead to the abnormal proliferation of renal stem cells (metanephric blastema). WT serves as a paradigm for understanding the relationship between loss of developmental control and gain of tumourigenic potential. In particular, loss of function of tumour suppressor genes has been implicated in the development of WT, and the Wilms' tumour suppressor gene WT1 (at chromosome 11p13) was the second tumour suppressor gene to be cloned, after the retinoblastoma gene RB-1. WT1 plays an essential role in kidney development, but is mutated in only approximately 20% of WTs, which suggests that further lesions and genetic loci are involved in Wilms' tumourigenesis. Other chromosomal regions associated with WT include 7p, 11p15, 16q and 17q. Although many of these loci probably contain tumour suppressor genes, imprinted genes (genes showing expression of only one parental allele) and oncogenes have also been implicated in WT. Some loci have been shown to be associated with particular clinical outcomes, suggesting that they might be used to determine prognosis, and especially to identify poor prognostic subgroups that can be targeted for aggressive and/or novel therapies.