The notion of endangerment stands at the heart of a network of concepts, values and practices dealing with objects threatened by disappearance, and with the devices, such as archives, catalogues or databases, aimed at preserving them. It thus opens the way for examining the construction of data deemed significant, the kind of knowledge such data constitutes, the structures into which it is organized, the affects that permeate it, and the moral and epistemic values it incarnates. Protecting the endangered and memorializing the extinct assume that the objects to be safeguarded or remembered are valuable; these are often associated with a supposedly natural or original state, sometimes (as in the case of dreams) with a condition of primeval authenticity. Architectural patrimony conserved in photographs, extinguished species in museum displays, or dead dialects in recordings nurture nostalgia for a more diverse world, and may give rise to resuscitation fantasies; together with dramatized depictions of imperiled places and organisms, they dynamize tensions between risk and heritage, and acquire political valence inside and outside science. The notion of endangerment transforms natural objects into cultural ones. Its centrality to projects for the protection of languages, the preservation of biodiversity, the defense of architectural patrimony, and much more seems to have crystallised around the mid-nineteenth century in different European and American lands. Such international scope and global chronological coincidence will contribute to frame the research of the working group into the various dimensions of the history and cultures of endangerment and its consequences.