The human body consists of hundreds of cell types, all originating from one fertilized egg. During the embryonic and foetal periods, the number of each cell type increases dramatically. The cells mature and become specialised to form the various tissues and organs of the body. Large numbers of cells are also formed in the adult body. Cell death is a normal process, which runs parallel to this generation of new cells. It takes place in both the foetus and adult, thereby maintaining the appropriate number of cells in the tissues. This delicate, controlled elimination of cells is termed programmed cell death or apoptosis. This year's Nobel laureates in Physiology or Medicine (Sydney Brenner, H. Robert Horvitz and John E.Sulston) have made seminal discoveries relating to the genetic regulation of organ development and programmed cell death. The nematode Caenorhabditis elegans was used as an experimental model system for following cell division and differentiation from the fertilized egg to the adult. The laureates have identified key genes that regulate organ development and programmed cell death and have also shown that corresponding genes exist in higher species, including humans. The discoveries are important for medical research and have shed new light on the pathogenesis of many diseases.