The U.S. Supreme Court’s opinion in Weyerhaeuser Co. v. U.S. Fish & Wildlife Service raises important questions about the scope of the Endangered Species Act’s (ESA’s) protections for critical habitat. Foremost among them is a question one might think was long settled: what is “habitat”? In a short ruling, the Weyerhaeuser Court opined that “critical habitat” must first be “habitat,” but it did not attempt to define exactly what habitat is or how much deference the U.S. Fish and Wildlife Service should get on what is both a biological and policy question. The Court also sidestepped whether currently unoccupied “habitat” must in fact be “habitable” at the time of designation as critical habitat. The task of defining “habitat” now falls to the ESA’s implementing agencies or to the U.S. Congress.  This Comment proposes to define habitat in a way that is consistent with the intent of the ESA, reflects the best available science, is operationally workable, and also broad enough to account for species’ needs.