Over the last decade the molecular genetic revolution has gathered pace. It is now in full swing and generating an avalanche of new information on disease processes at a molecular level. As yet the impact on the ward and in the clinic is negligible the gap between new understanding and practical benefit is often wide. Sceptics point out that the management of sickle cell disease has hardly changed in the 40 years since the basis of this, the first 'molecular' disease, was understood. But to quote ex-President Reagan, the basis of whose own disease, Alzheimer's, will soon be completely unravelled by molecular genetic techniques: 'you ain't seen nothing yet'. Medicine really is on the verge of the biggest revolution since William Harvey founded modern physiology, and the impact will be greatest in the area of paediatrics. 1,2 It is not just that we will understand every detail of what goes wrong in a vast collection of individually rare 'genetic' diseases. Perhaps more importantly, we will understand the genetic contribution to aetiology in a host of common 'multifactorial' disorders such as asthma, diabetes mellitus, epilepsy, pyloric stenosis, bed-wetting (yes, bed-wetting: a gene for dominant inherited nocturnal enuresis, ENUR1, has been mapped to, of all chromosomes, number 13), attention-deficit hyperactivity disorder, and obesity. How has this revolution come about? Furthermore, what has been achieved so far, and what does the future hold? For those paediatricians for whom the letters DNA are a prompt to decide whether another appointment should be sent rather than the acronym for the magic molecule at the heart of this revolution, this brief review attempts to answer these questions. A reminder of some of the basic concepts in molecular genetics is provided first a guide to the