This article is about the growing field at the intersection of environmental and constitutional law. Thirty years ago, constitutional issues rarely arose in environmental law. Nowadays, nearly two in three federal environmental, energy and land use cases are decided on constitutional grounds invoking no fewer than 18 issues. These include the extent to which Congress can regulate activities that are either traditionally intrastate or not inherently economic in nature (the Commerce Clause), preempt state causes of action (Supremacy Clause), and prescribe state functions (10th Amendment) or subject them to federal actions (11th Amendment). Other issues include whether states can burden interstate commerce (Dormant Commerce Clause) or must compensate for regulatory "takings" (5th and 14th Amendments); or whether citizens may enforce environmental laws (Take Care and Case and Controversy Clauses). Constitutional environmental law is emerging at the state and international levels. Two dozen states have constitutional provisions that address environmental matters. One hundred and thirty countries have constitutions that address its nation's environment, 70 of which provide individuals with some degree of what might be called "fundamental" rights to a "clean," "healthful" or "favorable" environment. The intersection of constitutional and environmental law also influences real estate, land use, administrative, torts, civil procedure, workplace safety, tax and international law. Transition in the Supreme Court makes it all the more likely the Constitution will continue to play an essential role in environmental law in areas involving congressional authority, federalism, property rights and limits on judicial review.