 In this entry, we have consider three philosophical issues in conservation biology. First, we began with the ontology of the discipline—what is biodiversity? We examined three proposals: biodiversity should be multidimensional, it should be operationalized, and that the concept should be eliminated from conservation practice. Second, we considered several different important cases studies in the science—modeling northern spotted owls, the equilibrium model of island biogeography and its impact on the SLOSS debate, and systematic conservation planning. What is striking is how traditional philosophical questions appear to disappear or be reoriented in recent work. For example, though there are theories and models, the core theoretical structures are different from other sciences since it involves various conventions and algorithms. Third, we consider the axiological foundations of the discipline. Many conclude that the sciences involve considerations of value. However, conservation biology was thought to be founded on the intrinsic value of ecological systems though the recent new conservationists have challenged this foundation as resting on soft sand. Critics, of course, have come out in full force arguing that this critique misses its mark and the foundations, though apparently shaky, are only apparently so.