One of the principle aims of epidemiology is the identification and understanding of risk factors for various diseases. Based on such knowledge effective intervention strategies can be developed and evaluated. In the case of diabetes mellitus, most of the excess morbidity and mortality is related to the chronic complications of the disease rather than to the acute problems that accompanies high or low blood sugars. In these two lectures, the epidemiology of these complications and their risk factors will be described with a particular focus on how the findings influence clinical practice. Lecture one examines the large vessel (macrovascular) complications and gives an overview of the small vessel (microvascular) complications, with a particular focus on eye complications (retinopathy). Lecture two examines the kidney (nephropathy) and nerve (neuropathy) complications and reviews various prevention trials.