Embryonic kidney is ideal for the study of epithelial morphogenesis--rich in developmental processes, accessible and clinically important. Interactions between mesenchyme and the epithelium invading it cause the epithelium to differentiate and branch to form the collecting ducts, and the mesenchyme to epithelialise into excretory nephrons. Molecular analyses reveal complex programmes of expression of transcription factors and morphoregulatory molecules, correlating with induction and morphogenesis. In developing nephrons especially, the detailed timing of this programme is now being explored and the functions of some molecules involved have been tested by inhibition experiments. New pharmacological methods of induction may also prove useful in solution of the mechanisms by which a kidney is formed.