Abstract: A review of the conservation biology literature reveals pervasive uncertainty about the effects of conservation policies on the persistence of populations of endangered species. This uncertainty about population persistence is of two types. Loosely speaking, certain features of the world that bear upon population persistence are uncertain, but are drawn from known distributions. Other features of the world that bear upon population persistence are uncertain, and are drawn from unknown distributions. This paper develops a systematic way to deal with these two types of uncertainty when selecting a conservation policy. In particular, we extend the results of Gilboa and Schmeidler (1989) to the problem of managing the extinction of a single species, and to the more general problem of managing biodiversity.