For many countries, the IUCN Red List of threatened species is a central instrument in their work to counteract loss of biodiversity. This article analyzes the development of the Red List categories and criteria, how these categories and criteria are used in the construction of global, national, and regional red lists, and how the red lists are employed in policy work. A central finding of the article is that this mix of actors implies many different forms of boundary work. This article also finds that the Red List functions as a portable representation, that is, a context-independent instrument to represent nature. A third finding is that the Red List functions as a link between experts and policy makers. Thus, the Red List is best understood as a boundary object and hybrid practice where the credibility of scientific assessment and a specific policy is mutually strengthened.