Population-based epidemiologic studies address 3 principal questions: how many people are affected by the disease (prevalence); who will develop this disease over time (incidence), and who is at risk for the disease and why they are at risk (analyses of risk factors). Over the past few decades, findings from population-based studies of ocular diseases have provided such information that guides clinical care and allow policy makers to design disease-screening programs and related public health strategies to minimize the impact of eye diseases in different communities.