Abstract This article is a synthesis of the major elements of a sociolinguistic theory presented by Jean Le Dû and Yves Le Berre in their recent book, Métamorphoses, Trente ans de sociolinguistique à Brest (1984–2014). Given that both authors come from native Breton-speaking families in Western Brittany and have experienced the language shift to French first-hand, they provide a unique, inside view of the process as well as the reasons Breton speakers opted in favour of French. The sociolinguistic concepts they have imagined provide highly useful tools that highlight the inseparable bond between language and the social, political and economic forces that govern our choices. More specifically, they point out that the “Breton language” is splintered into as many varieties as there are social and geographic entities in western Brittany. For this reason, it should not be viewed as a monolithic entity. Far from “reviving” or “saving” the language, the authors argue that the recent creation of a phonologically, grammatically and lexically unified Breton norm is often so distant from the vernacular language that it has provoked a new form of diglossia which failed to reverse the break in the transmission of the natural language. The book provides tremendous insight into the complex issues which lead people to shift to another language. Language planners and scholars working on similar endangered language situations and who want to understand the mechanisms at work (and thus hopefully have some success in their endeavours) would do well to take heed of their experience.