Although information about the precise world distribution and frequency of the inherited hemoglobin disorders is still limited, there is no doubt that they are going to pose an increasing burden on global health resources in the future. Their high frequency is a reflection of natural selection combined with a high frequency of consanguineous marriages in many countries, together with an epidemiological transition; whereby, as public health measures improve in the poorer countries of the world, more babies with these disorders are surviving to present for treatment. The inherited disorders of hemoglobin are by far the most common monogenic diseases. Here we will discuss the world distribution of these conditions and what is known about the mechanisms that have led to their extremely high frequency in many tropical countries. An estimate of the annual births of the important hemoglobin disorders is shown in Table 1.