First published in 1967. The problems of theoretical phonology are among the most controversial in linguistics. This monograph is a step towards an adequate logical reconstruction of phonological theories and is mainly concerned with Z. S. Harris’ structuralist theory, one of the principal phonological theories of the present day. Topics covered in the work include almost all essential problems of theoretical phonology. The author establishes a set of basic concepts which define almost all other concepts of phonology, and gives an axiomatic characterisation of these concepts. The notion of a unit-length segment is analysed and defined, and a precise formulation of the principles of distribution is given. The author offers a formal analysis of the notion of a phoneme, and finally formulates and discusses fundamental hypotheses of phonology.