16 2.1 Introduction The conservation of biodiversity is constrained by limited resources, and hence our investments in conservation must be prioritized. Spatial conser- vation prioritization is the process of using spatial analysis of quantitative data to identify locations for conservation investment. The process is regarded as more systematic, rigorous, and accountable than the opportunistic allocation of conservation funds (Margules and Pressey 2000). Priority areas can be identified not only for protected area establish- ment but also for investment in a variety of con- servation activities, ranging from fire management and invasive species control, to land restoration (Wilson et al. 2007) (Chapters 3 and 12). Since its origin in the early 1980s, the field of spatial conser- vation prioritization has influenced planning proc- esses, the development of conservation policies, and the expansion of protected areas. As a conse- quence, the number of publications concerning the development and application of spatial prioritiza- tion techniques has increased rapidly over the past two decades (Chapters 1 and 19). With increasing interest in quantitative tech- niques and tools, a plethora of principles and terms surrounding the field of spatial conservation priori- tization has arisen. This is to be expected with any expanding field, particularly one which extends a range of disciplines and sectors. Terms such as complementarity, comprehensiveness, representa- tiveness, and irreplaceability have been reviewed elsewhere (Margules and Pressey 2000; Possing- ham et al. 2006; Margules and Sarkar 2007). Prin- ciples such as cost efficiency, threat, adequacy, and vulnerability are receiving increasing attention. The terms and principles reviewed in this chapter are used throughout this book and many are described mathematically in Chapters 3–5. In this chapter we define these principles and terms in an accessible manner, explore their meaning using examples and case studies, and provide operational definitions to inform future prioritization analyses. 2.2 Key principles of spatial conservation prioritization 2.2.1 Comprehensiveness, representativeness, and surrogacy – sampling the full range and characteristic examples of biodiversity In simple terms, a comprehensive network of pri- ority areas is one that includes a portion of every biodiversity feature. More broadly the notion of comprehensiveness in conservation prioritization implies sampling the full range of biodiversity taking into account composition (e.g. species and genetic diversity), structure (e.g. habitat types), and function (e.g. recruitment and dispersal processes) (Noss 1990). In addition, each biodiversity feature protected should ideally be representative of that feature. While spatial conservation prioritization requires spatial information about the distribution of biodi- versity, our knowledge of the biodiversity on earth is remarkably poor. This problem has two key ele- ments, termed ‘Linnean’ and ‘Wallacean’ shortfalls (Whittaker et al. 2005). The Linnean shortfall refers to our lack of knowledge of how many, and what kind, of species there are. The Wallacean shortfall CHAPTER 2 Fundamental Concepts of Spatial Conservation Prioritization Kerrie A. Wilson, Mar Cabeza, and Carissa J. Klein