The use of transgenic animals represents a powerful tool with which to address the role of particular gene products in vivo. Recent technical and biological advances have simplified the process of creating both transgenic mice and null-mutation mice. Increasing numbers of genetic control elements are available to direct transgene expression to particular renal cell types and to enhance the consistency of expression. These approaches have contributed significantly to our understanding of renal development and pathogenesis, in particular in the following areas: the roles of various oncogenes, homeobox genes, and growth factors in renal development and the pathogenesis of cystic renal diseases; the contribution of systemic and local expression of the renin-angiotensin system to blood pressure control; the role of growth factors and cytokines in progressive glomerular disease; the role of viral proteins in the pathogenesis of glomerular and tubular disease; and mechanisms of immune-mediated renal disease.