Labov's empirical vindication of the neogrammarian hypothesis, his critique of functionalist accounts of sound change, and his formal principles governing vowel shifts, mergers and splits are major contributions towards solving the constraints problem in historical phonology. I explore their implications for phonological theory, and show that they connect with recent developments in Optimality Theory in mutually illuminating ways.Labov begins his landmark Principles of Linguistic Change with a volume on internal factors governing sound change (Labov 1994) that focuses on the CONSTRAINTS PROBLEM (Weinreich, Labov and Herzog 1968): what changes are possible for a language in a given state? On the basis of typological data on sound changes and his research on ongoing change in English, he puts forward a set of principled constraints and generalizations, which the field will be building on for decades to come. Here, I take up each of the four major theoretical themes of the bookthe regularity hypothesis, mergers and splits, chain shifts, and functionalismand explore the significance of Labov's findings for generative phonology, and the ways in which recent developments in phonological theory might help advance Labov's research program.In a longer essay I would have tried to trace the remarkable trajectory from Labov