ABSTRACT: The US Endangered Species Act (ESA) allows protection of any species that is at risk in all or ‘a significant portion of its range’ (SPOIR). Because this provision is open to many possible interpretations, the agencies responsible for implementing the ESA recently published a SPOIR policy. The policy is based on a framework we developed that asks a simple question: ‘If the portions of the range that are currently at risk were lost, would the entire species, at that point, be threatened or endangered?’ If so, the portion of the range is significant. Some commentators have argued that the policy departs from goals the ESA was originally intended to accomplish. We disagree; biologists and managers struggling to implement provisions of the ESA in complex, real-world situations need practical guidance, and we believe our framework provides that. In particular, it avoids as much as possible normative considerations in evaluating ‘significance’ in terms of human values; instead, we focus on significance to the species, which is consistent with the ESA focus on preventing extinctions, as well as with the mandate that listing determinations be based ‘solely’ on scientific information. However, we agree with some critics that a crucial factor in implementation of the policy will be how historical versus current concepts of range are reconciled. We believe that historical distribution and abundance are important, not as specific restoration goals, but as reference points that characterize conditions under which we are confident the species was viable.