This review provides an overview of the types of information epidemiological research can provide and how these data can be used. The aim is to provide the readers with basic epidemiological skills to allow them to read critically scientific articles and to gain proficiency in communicating about epidemiological research. All examples in the review are drawn from the ophthalmic literature. The first part of the review is relatively conceptual and focuses on epidemiological theory, including case definition, measures of the burden of disease, sampling and the interpretation of results. In the second part different study designs are described--specifically, cross sectional surveys, cohort studies, case-control studies, and randomised controlled trials, and the strengths and limitations of each highlighted.