Normal development of the kidney is a highly complex process that requires precise orchestration of proliferation, differentiation, and apoptosis. In the past few years, a number of genes that regulate these processes, and hence play pivotal roles in kidney development, have been identified. The Wilms' tumor suppressor gene WT1 has been shown to be one of these essential regulators of kidney development, and mutations in this gene result in the formation of tumors and developmental abnormalities such as the Denys-Drash and Frasier syndromes. A fascinating aspect of the WT1 gene is the multitude of isoforms produced from its genomic locus. In this review, our current understanding of the structural features of WT1, how they modulate the transcriptional and post-transcriptional activities of the protein, and how mutations affecting individual isoforms can lead to diseased kidneys is summarized. In addition, results from transgenic experiments, which have yielded important findings regarding the function of WT1 in vivo, are discussed. Finally, data on the unusual feature of RNA editing of WT1 transcripts are presented, and the relevance of RNA editing for the normal functioning of the WT1 protein in the kidney is discussed.