Citation indexes are based on the principle of authors citing previous articles of relevance. The paper demonstrates the long history of citing for precedent and notes how ISI's citation indexes differ from Shepards Citations. The paper analyses some of the criticisms of citation counting, and some of the uses for which citation analysis has been employed. The paper also examines the idea of the development of an Acknowledgements Index, and concludes such an index is unlikely to be commercially viable. The paper describes a citation study of Eugene Garfield, and concludes that he may be the most heavily cited information scientist, that he is a heavy self-citer, and that the reasons why other authors cite Garfield are different from the reasons why he cites himself. The paper concludes that citation studies remain a valid method of analysis of individuals', institutions', or journals' impact, but need to be used with caution and in conjunction with other measures.