Glucose 6-phosphate dehydrogenase (G6PD) is the first enzyme of the pentose phosphate pathway. It exists in over 250 variants which are divided broadly into five classes on the basis of residual enzyme activity and clinical manifestations. The variants with reduced activity result in G6PD deficiency. This is inherited as an X-linked recessive disorder and occurs at a much higher frequency in the male than in the female. Most G6PD deficient individuals show no clinical abnormality under normal conditions, but acute hemolytic crisis may occur. Several nonhemolytic abnormalities occur as well in G6PD deficients at a higher frequency than in nondeficients. Several nonhemolytic abnormalities occur as well in G6PD deficients at a higher frequency than in nondeficients. The genetic, pathophysiologic, and therapeutic aspects of G6PD deficiency are presented; and the possibility of genetic counselling, care in blood banks, and benefits of education the G6PD deficients are discussed.