Introduction In the discussion about biodiversity loss and conservation (cf. Wilson 1988; McNeely et al. 1990; Watson et al. 1995; Millennium Ecosystem Assessment 2005), the two issues of (i) quantitative measurement of biodiversity and (ii) its economic valuation play a major role. Concerning the first issue, there exists an extensive ecological literature (see, for example, Pielou 1975; Magurran 1988; Purvis and Hector 2000) to which, lately, economists have made important contributions (e.g. Solow et al. 1993; Weitzman 1992, 1993, 1998; Weikard 1998, 1999, 2002; Nehring and Puppe 2002, 2004). Concerning the second issue, there also exists an extensive and still growing literature (e.g. Hanley and Spash 1993; Pearce and Moran 1994; Smith 1996; Goulder and Kennedy 1997; Nunes and van den Bergh 2001; Freeman 2003). The conventional wisdom on the relation between these two issues seems to be that the quantitative measurement of biodiversity is a value-free task which precedes the valuation of biodiversity. In contrast to this view, I shall argue in this chapter that the measurement of biodiversity requires prior value judgements about biodiversity and its role in ecological-economic systems. The argument proceeds as follows. Section 2 briefly discusses why and how biodiversity can be thought of as an economic good, so as to provide a background for the discussion of the measurement of biodiversity. Section 3 then surveys different ecological and economic measures of species diversity. A conceptual comparison of those reveals systematic differences between the two.