This paper outlines the influence of federal, state and international constitutional law on measures to protect species' habitat, with special focus on those taken under section 4 of the federal Endangered Species Act. For example, critical habitat designation engenders a host of constitutional questions, including whether it falls within Congress' authority under the Commerce, Property, General Welfare and Takings Clauses of the U.S. Constitution. It also involves an examination of the extent to which habitat designations are supportable or assailable under state constitutional law, as well as the constitutional law of nations.