In phonology, as well as in syntax and semantics, we find three types of variation: (i) quantitative variation: a grammatical regularity surfaces sometimes categorically, sometimes quantitatively; (ii) lexical variation: a grammatical regularity emerges in some (classes of) lexical items, but not in others; (iii) variation in interaction: some grammatical regularities interact, others do not. I propose that these three phenomena are different aspects of the same problem and outline an optimality-theoretic solution that implies the presence of principled variation within an individual's grammar. I illustrate the approach by examining phonological variation in 25 regional dialects of Finnish.