Data on the prevalence, magnitude and causes of blindness and severe visual impairment in children are needed for planning and evaluating preventive and curative services for children, and for planning special education and low vision services. Prevalence data can be obtained from a variety of different sources, each of which has limitations. The available data suggest that there may be a ten-fold difference in prevalence between the wealthiest countries of the world and the poorest, ranging from as low as 0.1/1000 children aged 0-15 years in the wealthiest countries to 1.1/1000 children in the poorest. In this paper, the available data are reviewed and the epidemiological methods and findings are discussed.