There is a growing awareness that a fruitful cooperation between the (diachronic and synchronic) study of language variation and change and work in phonological theory is both possible and desirable. The study of language variation and change would benefit from this kind of cooperation on the conceptual and theoretical levels. Phonological theory may well profit from a greater use of what is commonly called 'external evidence'. This volume contains contributions by outstanding representatives from the more data-oriented fields and phonological theory. They discuss possibilities and problems for a further integration of both areas, by considering questions such as where and to which extent the two may need each other, and whether there is a need for an interdisciplinary conceptual framework and methodology. Attention is also paid to questions regarding the cause and actuation, linguistic constraints and the internal spread of linguistic change, as well as to possible and impossible processes of language change.