Abstract: This paper is part of an effort to compile a vocabulary for Degrowth. Such a collection of concepts for the paradigm is important for the creation of a common language and knowledge base. This article is the first one, which tries to offer a universal definition of resource peaks and explains de main basics behind such phenomena. Peak-Oil is one of the most central elements in the biophysical arguments against the possibilities of future economic growth. After crossing the peak oil (and other resource peaks) the physical expansion of the economy is no longer possible and a managed socially sustainable decent has to be designed, even if complex and difficult.