The ‘standard theory’ of Chomsky and Halle has dominated phonology in recent years. It has been subject to modification and to criticism but not of a really fundamental kind. Dr Foley does here offer a fundamental criticism and a genuine theoretical alternative. He argues that transformational phonology, like previous phonological systems, is primarily concerned with the description of superficial sound changes and not with the underlying processes and rules; it is perhaps more accurately termed ‘transformational phonetics’ for that reason. A theoretical phonology, he argues, will consist of a system of phonological elements, a set of universal rules relating these elements and a set of principles governing the operation of the phonological rules. The basic phonological elements are therefore defined not by physical acoustic or articulatory parameters, but by their participation in rules. Such a theory is developed here and illustrated in the analysis of various phonological problems. It is shown that many apparently diverse phenomena can thus be connected and explained and so be subject to genuinely scientific enquiry.