This article discusses definitions of index and indexing and provides a systematic overview of kinds of indexes. Theories of indexing are reviewed, and the theoretical basis of both manual indexing and automatic indexing is discussed, and a classification of theories is suggested (rationalist, cognitivist, empiricist, and historicist and pragmatist theories). It is claimed that although many researchers do not consider indexing to be a theoretical issue (or consider it to be a field without theories) indexing is indeed highly theory-laden (and the idea of atheoretical indexing is an oxymoron). An important issue is also the subjectivity of the indexer, in particular her socio-cultural and paradigmatic background; for example, if authors of documents are the best indexers of their own documents. The article contains a section about the tools available for indexing in the form of the indexing languages and their nature. It is concluded that the social epistemology first proposed by Jesse Shera in 1951 provides the most fruitful theoretical framework for indexing.