Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the commonest enzyme disorder of human beings and a globally important cause of neonatal jaundice, which can lead to kernicterus and death or spastic cerebral palsy. It can also lead to life-threatening haemolytic crises in childhood and at later ages, by interacting with specific drugs and with fava beans in the diet. The complications of G6PD deficiency can largely be prevented by education and information, and neonatal jaundice can be successfully treated by phototherapy, a cheap and simple approach suitable for use in primary health care. This update describes developments in the methodology for characterizing G6PD deficiency, recent knowledge of the factors that can cause haemolysis, community approaches for prevention of haemolytic crises and neonatal jaundice, and the implications of recent advances at the DNA level.