By definition, an indefinite inner product space is a real or complex vector space together with a symmetric (in the complex case: hermi- tian) bilinear form prescribed on it so that the corresponding quadratic form assumes both positive and negative values. The most important special case arises when a Hilbert space is considered as an orthogonal direct sum of two subspaces, one equipped with the original inner prod- uct, and the other with -1 times the original inner product. The subject first appeared thirty years ago in a paper of Dirac  on quantum field theory (d. also Pauli [lJ). Soon afterwards, Pontrja- gin  gave the first mathematical treatment of an indefinite inner prod- uct space. Pontrjagin was unaware of the investigations of Dirac and Pauli; on the other hand, he was inspired by a work of Sobolev [lJ, unpublished up to 1960, concerning a problem of mechanics. The attempts of Dirac and Pauli to apply the concept and elemen- tary properties of indefinite inner product spaces to field theory have been renewed by several authors. At present it is not easy to judge which of their results will contribute to the final form of this part of physics. The following list of references should serve as a guide to the extensive literature: Bleuler , Gupta [lJ, Kallen and Pauli [lJ, Heisen- berg [lJ-[4J, Bogoljubov, Medvedev and Polivanov [lJ, K.L.Nagy [lJ-, Berezin [lJ, Arons, Han and Sudarshan , Lee and Wick [1J.