Editor's Summary: Prof. Jonathan Nash suggests that it may be possible to construct a viable development rights trading regime that would protect ecosystems and endangered species by relying on lessons from other trading programs, such as air emissions credits. Virginia Albrecht finds the analogy to air emissions credits weak because it fails to take into account the private-property rights inherent in land, the many competing demands on land use, and the fungibility of air. Michael Bean believes there are some lessons to be learned by considering development rights trading schemes, but that ultimately they are unlikely to succeed given current limitations to our knowledge about species and habitat.