This paper discusses the interpretation of data on two types of phonological change: change in language over time in the culture, and change in the development of the individual speaker; and examines the position that these two sorts of change interact in a certain way in relation to phonological structure. If one conceives of phonology as a finite set of ordered rules which relate surface syntactic structure to the phonetic descriptions of a language, there are three obvious potential ways for it to change: a rule can be added or deleted, changed, or reordered. Assumed is that phonological rules present certain stable characteristics of the individual's perceptual-motor operations and provide the natural central units for the processing of language sounds. The changes present in"phonological drift" have been thought to represent the typical contribution of children during initial language acquisition; they are not found in adults, who are less able to reconstruct basic aspects of their phonological system. The typically adult form of sound change, rule addition or deletion, is not found in children, who are built so that they organize rather than add. Adults have much more greater control over the form of the language. The program of psycholinguistic investigation sketched here for phonology needs to be carried out for other parts of the grammar as well. (Author/AMM)