Citation indexing is one of the means of producing an effective index by the effective use of computer for arranging and reformatting entries. The input to this indexing system comprises the references of recent articles published in the core journals of any discipline. For each article, a list of works (documents) that it refers to are also compiled. In other words, a citation index lists 'cited' documents, together with a list of those items that have cited them (i.e., citing articles). The most important example of citation index is that of Science Citation Index (SCI) produced by Institute of Scientific Information. The specialty of this type of indexes is that given one document (cited) in a field, the searcher should be able to trace other related documents. Citation links provide a valuable additional approach to the identification of related documents. This type of indexing could become feasible only through the use of computers. Manual compilation may take a long time and may not be worth attempting. ISI also brings out Social Science Citation Index and Arts and Humanities Citation Index.