G6PD deficiency is the most common enzymopathy in the world. The highest frequency values are found in tropical Africa, in the Middle East, in some areas of the Mediterranean, in tropical and sub-tropical Asia and in Oceania. This genetic defect shows sex linked inheritance and a marked heterogeneity. At least 400 abnormal variants with different biochemical characteristics and about 100 diverse mutations have been identified. In most cases the phenotypic expression is a marked decrease in erythrocyte G6PD activity. The most common clinical consequences are neonatal jaundice and sporadic haemolytic crises caused by a number of drugs, by infections or by ingestion of fava beans. A few cases of chronic non-spherocytic haemolytic anaemia associated with rare molecular variants have been reported. Early diagnosis, education and epidemiologic surveillance have been proved to be cornerstones in the prevention of the haemolytic disease. Therefore they should be taken into account in the national health programmes, especially in the countries with high prevalence rates.