When Eugene Garfield introduced the concept of a citation index for the sciences in 1955, he emphasized its several advantages over traditional subject indexing. Since a citation index records the references in each article indexed, a search can proceed from a known work of interest to more recently published items that cited that work. Moreover, a search in a citation index, either forward in time or backward through cited references, is both highly efficient and productive because it relies upon the informed judgments of researchers themselves, reflected in the references appended to their papers, rather than the choices of indexing terms by cataloguers who are less familiar with the content of each publication than are the authors. Garfield called these authors “an army of indexers” and his invention “an association-of-ideas index.” He recognized citations as emblematic of specific topics, concepts, and methods: “the citation is a precise, unambiguous representation of a subject that requires no interpretation and is immune to changes in terminology.” In addition, a citation index is inherently cross-disciplinary and breaks through limitations imposed by source coverage.