The evolution of phonological theory over the past fifteen years or so mirrors in important respects that of syntactic theory. The common evolutionary characteristic is the emergence of modularity. As is frequently noted following Chomsky (1986: ch. 3), in syntax, rich systems of rules have been supplanted by a relatively small number of discrete sub-theories, such as the theories of thematic relations, Case, Binding, Government, and the theory of empty categories. Characteristically, each sub-theory regulates one specific aspect of syntactic structure, at one or more levels of representation, for example the distribution of overt noun phrases at S-structure. The emergence of the sub-theories reflects a natural shift in investigative focus. Just as studying the facts of language from a systematic and formal perspective led to the discovery of generalizations of fact, originally expressed as ‘rules’, so the study of the rules themselves led to the discovery of higher-order generalizations, expressed by the various conditions or principles that make up the contemporary sub-theories. Although Chomsky (1986:70ff.) lists several important contributors to the development of the new perspective, in the mind of most syntacticians, a watershed event in this evolution was Chomsky's own ‘Conditions on transformations’ (1973). To the extent that this development of syntactic theory is a natural one towards deeper understanding, a comparable one is expected in phonological theory. Although the ‘modularity’ of phonology is less frequently noted and identification of a single watershed event is perhaps more difficult, there are clearly several ‘modules’ or sub-theories that have emerged in post-SPE history – three in particular.