The title of this chapter brings together two of the most critical issues raised by the relation of human beings to the natural world. At first, the rationale for bringing them together may be unclear. The threats human activities pose to biodiversity are among the most urgent concerns of environmentalists, but of course, biotechnology accounts for only a narrow range of these threats. Similarly, of the urgent ethical questions posed by biotechnology only a few are directly relevant to biodiversity. Nevertheless there is significant overlap between ethical concerns connected with biodiversity and those connected with biotechnology. This overlap appears, for example, in efforts to enlist biotechnology in the preservation of biodiversity, but the most poignant concerns arise with the prospect of impact on biodiversity from transgenic organisms. Opponents of genetically modified organisms regularly point to the potential effects of these organisms on biodiversity as a reason for prohibiting or strictly regulating research on and application of transgenics. This chapter attempts to evaluate these claims by a careful consideration of the issues they raise in the context of current scientific research. The chapter is structured by an analysis of the factual and normative issues at stake. In order to evaluate the claims regarding transgenic research and biodiversity we must ask two preliminary questions. The first preliminary question is factual; it has to do with the potential impact of genetically engineered organisms on biodiversity. There is widespread concern about the effects such organisms may have on biodiversity, but what do we know about the likelihood of such effects? Uncertainty about these effects themselves and about the future direction of biotechnology makes this a difficult question, but we nevertheless attempt to answer it in Section 7.1.2 below in relation to the most likely foreseeable developments in transgenics.