Young people's language use and cultural orientations have been observed to be fluid in the sense that they transgress and blend sets of linguistic and cultural resources, but it has also been observed how such fluid language use over time become recognized as distinct youth styles – a process involving a sense of linguistic fixity. This chapter interrogates these bidirectional linguistic processes of fixity and fluidity, which may be particularly heightened in the practices of young people, but are central to sociolinguistic theory and research beyond youth studies. The chapter reflects on the evidence, rationale and epistemological contribution of popular conceptions of linguistic fluidity, and critically discusses widespread (con)fusions of descriptive, ontological, pedagogical and political purposes in recent theoretical approaches. Based on examples from linguistic ethnographic research on youth, the chapter suggests as a possible future direction an approach grasping both linguistic fixity and fluidity, inspired by the notion of enregisterment and combining investigations of participant understandings and situated language use (through ethnographic accounts and interaction analysis).