The U.S. Constitution propels the majority of environmental litigation. Thirty years ago, constitutional issues seldom arose in environmental law. Nowadays, nearly two in three federal environmental, energy and land use cases are litigated on constitutional grounds. Such cases implicate approximately twenty constitutional principles involving federalism, separation of powers and individual rights. Constitutional issues in environmental litigation are torn from the headlines, from climate change to natural resource extraction. Accordingly, this chapter aims to contextualize constitutional litigation for environmental lawyers in five ways. Part One provides a brief background to environmental litigation in the United States. Part Two addresses how constitutional limits on judicial review influence environmental litigation, including Article III constitutional standing and the political question doctrine. Part Three examines constitutional limits on Congress’ authority to enact environmental laws, principally under the Commerce and General Welfare Clauses. Part Four explains how constitutional principles of federalism influence environmental litigation, including the 10th Amendment, 11th Amendment, and the Supremacy, Dormant Commerce, Treaty, and Compact Clauses. And finally, Part Five discusses the impact that constitutional jurisprudence addressing individual rights has on environmental litigation, including the Takings, Due Process and Equal Protection Clauses, and the First Amendment. Ultimately, this chapter concludes that Constitutional Law and environmental litigation are inextricably intertwined. Environmental lawyers in the 21st Century need to expand their expertise in statutory and regulatory law to include the constitutional features of judicial review, congressional authority, federalism, and individual rights.