A fundamental property of any ecosystem or habitat is the number of species it contains. Some wetlands contain large numbers of species; some contain few. Within a single wetland, there is similar variation, with some habitats being species rich and other species poor. Such patterns in diversity have long been of keen interest to ecologists (e.g. Williams 1964; Pielou 1975; May 1986; Huston 1994; Gaston 2000) and conservation biologists (Ehrlich and Ehrlich 1981; World Conservation Monitoring Centre 1992). The long-term goal of such work is to determine how many species occur in different parts of the world, and to uncover the factors that predict how many we will find in specified conditions. Since there are good reasons to fear that up to one-fourth of the world's species could disappear in the coming century, it is important to manage wild places, including wetlands, to maximize the number of native species found globally. When species are at risk of extinction – be they quill worts or tigers – managers must create and protect enough habitat to allow their populations to recover. Hence, the study and understanding of diversity occupies a central position in wetland ecology. The objective of this chapter is to survey the number wetland species found in major groups of organisms and then introduce you to some of the critical environmental factors that control them. Often only a couple of factors will determine whether a wetland supports many species or only a few.