Although most of the world's children live in developing countries and may be at high risk for disability, very little is known about the prevalence and causes of developmental disabilities in these countries. This paper discusses methodological difficulties contributing to this lack of knowledge, and provides an overview of what is known about the epidemiology of developmental disabilities in low-income countries. At least some forms of developmental disability appear to be more common in low-income countries than in wealthier countries, despite the probability of higher mortality among children with disabilities in low-income countries. For example, most studies of severe mental retardation in low-income countries report prevalences greater than 5 per 1,000 children, while prevalence estimates from industrialized countries are consistently below this. Major risk factors for developmental disabilities in some low-income countries include specific genetic diseases, a higher frequency of births to older mothers, consanguinity, and specific micronutrient deficiencies and infections. Trauma and toxic exposures are also important risk factors, but their contributions to the etiology of developmental disabilities in low-income countries are not well documented. Though many of the causes of developmental disabilities are understood and preventable, proven methods of prevention are not being fully implemented in developing countries. Epidemiologic studies are needed to raise awareness of the public health impacts of developmental disabilities in low-income countries and to provide a basis for setting priorities and designing efficient interventions. MRDD Research Reviews 2002;8:206–211. © 2002 Wiley-Liss, Inc.