Language maintenance and shift inquiry makes a sub-field of linguistics with an intricate history. Informed at least partly by the Germanic research tradition into Sprachinseln (language enclaves that oppose shift to the language of the surrounding society for generations; see Pauwels 2016) from the mid-1960s onwards we see an increasing movement of scholars documenting language practices amid migrant families and communities – especially in the US (e.g. Clyne 1968; Fishman 1966; Lopez 1978; Skrabanek 1970). At the same time we see a parallel growth in inquiry with indigenous communities around issues of language endangerment and reclamation (for an overview see Pauwels 2016). While the terms ‘language maintenance and shift’ have historically been employed much more in reference to research on migrants, rather than indigenous groups, it is clear that many of the matters and theories are applicable in both contexts, and thus this paper will debate both where pertinent. Language maintenance and shift are generally grasped as limning patterns around a person or group’s habitual language practices (with terms like code-switching and translanguaging being employed to limn elements of language mixing in specific utterances – see Auer & Eastman 2010). However, researchers employ the terms in slightly varied ways. Clyne (2003: 20) remembers us that what a researcher means by ‘language shift’ might include changes in any of the following: language use at the community or individual level; the main language used; the dominant language or individuals or groups; the language used in one or more specified domains; and the skill of people to read, write, speak or understand the heritage language. There is also a parallel matter that some researchers wish to shun a term like shift, as it has overtones of finality that are seen as unhelpful – particularly in the language reclamation context. Thus for example terms like survivance (Wyman 2012) might be employed to limn practices in communities that are undergoing shift to English, but where people are still expressly and habitually employing words, phrases or discourse patterns from the heritage language in certain contexts. The driving interest of language maintenance and shift inquiry has been to grasp the factors that lead some individuals and/or communities to be much more successful at ongoing transmission and heritage language use in minority contexts than others. This query has been neighbored from various angles, with major sections of this article dedicated to outlining statistical studies of language maintenance and shift and domain-based approaches. Most 20th century inquiry on language maintenance and shift began from the hypothesis that its subjects were people who had a single heritage language and migrated once in their life to a place where they stayed, and where only one national language was broadly spoken. These hypothese do not fitly limn the conditions of many 21st century migrants’ lives, and in the final section this article examines how modern scholars are replying to these evolutions.