Environmentalists consider invasions by exotic species of plants and animals to be one of the most serious environmental problems we face today, as well as one of the leading causes of biodiversity loss. We argue that in order to develop and enact sensible policies, it is crucial to consider two philosophical questions: What exactly makes a species native or exotic, and What values are at stake? We focus on the first of these two questions, and offer some preliminary suggestions with (...) regard to the second. Through a series of case studies, we show that it is not always clear whether a species is native or exotic. We identify five possible criteria that could be used for distinguishing natives from exotics. Rather than identifying one of these criteria as the 'correct' one, we suggest that the concepts of 'native' and ' exotic ' function more like what some philosophers have called cluster concepts. That is, there are several characteristics that are typical of native species, and a corresponding set of characteristics that are typical of exotic species. None of these characteristics is either necessary or sufficient for identifying a species as either native or exotic. We then identify several of the values that are at stake in dealing with exotic species, and we suggest that policies need to avoid being overly simplistic. (shrink)