Medical researchers often are faced with the challenge of estimating the total number of cases in a population based on incomplete samples. Because of a lack of explicit methods for determining if all cases have been counted, indirect methods for estimating the abundance of disease have been developed. Capture-recapture models are an indirect method of estimating population sizes that have been employed in recent epidemiological studies. These methods, derived from techniques developed for studies of animal abundance, estimate the true population size by evaluating the degree of overlap among incomplete lists of cases from existing data sources. Although intuitively appealing, the successful application of these methods is dependent upon a clear understanding of the biology of the disorder involved, the dynamics of the reference population, and the assumption and robustness of the specific models used. Failure to address these issues can lead to inaccurate and sometimes misleading results. This article describes some of the strengths and limitations of recapture techniques and provides the reader with a foundation from which to explore the methods in further detail.