In 2015, the American Library Association adopted the Resolution on the Importance of Sustainable Libraries; since then, other international organizations have been quick to go along with the proposal, reporting on the potential relationship between sustainability and libraries. However, such documents (which, in general, support the role of librarians in building “sustainable, resilient and regenerative” communities and making “sustainable decisions”) remain purely statements of intent… that include a handful of trendy topics in their paragraphs, and fall short of being credible action plans. It is worrying to note that, despite the seriousness and urgency of the discussion, these statements tiptoe around a crucial issue―sustainability―that, so far, has not been addressed in depth by library and information sciences. The following paragraphs are intended to confront the reader with the impossibility of unlimited growth in a finite biosphere, and are aimed at introducing the notion of sustainability and other concepts related to it—in particular “degrowth,” which remains ignored in many forums on sustainable development, including libraries. The article will also address the links that can be established among sustainability, activism, and libraries’ services, activities and policies. The ideas presented here are meant to serve as starting points, guidelines or major strands to help readers search through international bibliography on an issue in need of urgent attention.