In 1991, Dr. Alfred Sommer, the initial editor of this section, wrote that “geographic” and “preventive” ophthalmology share a common tradition and perspective, which explains the composition of this section. Both are firmly rooted in public health disciplines and public health pursuits; both are concerned with all aspects of an ophthalmic problem, from its root cause to its prevention and cure. A decade or more later, this is still true.

He emphasized that the careful study of individual patients would continue to provide important scientific insights. However, increasingly, the application of powerful epidemiologic tools to “groups” and “populations” or individuals has enhanced the ability to identify factors responsible for disease: from racial and genetic variations in the risk of chronic open angle glaucoma^1,2 and age-related maculopathy^3,4 to environmental^5,6 and dietary factors^7,8 that may be responsible for cataract. These same techniques provide rigorous means for evaluating treatment modalities and their long-term impact.^9,10,11,12 These approaches and their interpretation are discussed elsewhere in this text. All ophthalmologists can benefit from the review: it can improve the quality of research and, more importantly, the ability to interpret and assimilate reports published in the clinical literature.^13,14,15

At a time of increasing concern with health care costs, access, and quality, the “population” perspective to ocular morbidity and visual impairment has become even more compelling. For the first time, we have begun to develop truly representative estimates of the distribution and magnitude of visual impairment and blindness in the United States.^{16,17,18,19,20,21} Other chapters in this book lay the groundwork for understanding these issues of measures of visual function and the latest population estimates.