It is now widely accepted that diabetes is a heterogeneous disorder, that is, it comprises several different disease states each of which has a different underlying aetiopathogenesis (Cudworth, 1981). The evidence for this has slowly accumulated over many years, but it was not until the 1970s that the HLA data from both population and family studies provided definitive scientific evidence that the two major types of idiopathic diabetes, type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes, are genetically different. It also has to be borne in mind that frank diabetes or impairment of glucose handling can result from a large number of stressful events such as pregnancy, and can also be caused by hormonal factors and drugs. The net result in terms of metabolic abnormalities may well be indistinguishable from what is observed in overt idiopathic diabetes. A second fundamental point, which applies to all types of diabetes, is that diabetes per se is not inherited, rather it is the susceptibility to develop the disease which is transmitted. What remains enigmatic is the question of which environmental factors convert the underlying susceptibility into the overt state of clinical diabetes. Another point is that independent of the cause of diabetes, the same long-term complications may arise irrespective of the underlying aetiology of the glucose intolerance. It is also important to remember that diabetes expresses itself with considerable variability in different ethnic groups, and almost certainly different environmental factors operate to produce what looks like the same type of diabetes in different populations. Over the past few years there have been several attempts to reclassify diabetes. Any classification is likely to be controversial, but the recent WHO Expert Committee (1980) has gone a long way in clarifying current ideas. An extension of this concept is shown in Table 1 in an attempt to embrace some of the newer evidence which points to the heterogeneity which undoubtedly exists in both major types of diabetes. This chapter deals specifically with type 1 (insulin-dependent) diabetes. It is of great interest that this is predominantly a Caucasoid disease, being relatively rare in Blacks and in Mongoloid populations, even though