The study and formalization of intra-speaker variation within variationist sociolinguistics has followed a largely parallel history with generative phonology, always borrowing heavily from the generative theories of the day. More recently, structured probabilistic variation has become enshrined as a fact-to-be-explained by any theory of human sound systems in more mainstream phonology. This chapter outlines this parallel history of variation study from its origins in dialectology, the evolution of modern variationist sociolinguistics, and the development of more contemporary variation focused phonological theory, as well as critiques that have been posed over this history. The chapter reviews in considerable detail how the original notion of ‘variable rule’ was elaborated and complexified, and how variation is treated in constraint-based approaches. It concludes with a look towards the future of variation study that is incorporating more insights from psycholinguistics.