Citation data have become an increasingly significant source of information for historians, sociologists, and other researchers studying the evolution of science. In the past few decades elaborate methodologies have been developed for the use of citation data in the study of the modern history of science. This article focuses on how citation indexes make it possible to trace the background and development of discoveries as well as to assess the credit that publishing scientists assign to particular discoverers. Kuhn's notion of discovery is discussed. The priority dispute over the discovery of the AIDS virus is used as an example.