Bibliometrics has been increasingly applied to research evaluation in recent years. This article reviews studies which supply data to assess the validity and accuracy of citation indicators as a measure of performance. To illustrate the points raised, the article also examines results obtained by case studies on Nordic cancer and cardiovascular research. It concludes that citations measure recognition of a piece of work by the international scientific community, but are also influenced by publication structures, citation conventions and social relations in the scientific community. Citation analysis complements peer review by supplying evidence to assess its accuracy and reliability.