Conserving the biodiversity of a region requires mapping where different species can be found and then identifying conservation sites that are large enough and arranged in such a way that long-term persistence of the targeted species is maximized. Increasingly, such planning entails looking beyond political boundaries to include regions whose boundaries are defined ecologically, such as a mountain range and the adjoining coastal plain. Mathematical models and other quantitative approaches are key elements in the toolkits of modern conservation planners and managers, both in investigating the extinction risks faced by different species and in identifying suites of sites that can protect as much of a region's biodiversity as possible.