The Fourth Edition is updated to take account of new developments in the law, new regulations, and new cases. The most comprehensive updates are found in Chapter Four on the Clean Air Act and Chapter Five on the Clean Water Act. Chapter Four has been reorganized to provide professors and students with a new comprehensive section on climate change, including the EPA's many regulatory efforts to address greenhouse gas emissions from both mobile and stationary sources, critical U.S. Supreme Court decisions, and an overview of the Clean Power Plan, which is in litigation as the Fourth Edition goes to press. Chapter 5 includes a new approach to the "waters of the United States" element of Clean Water Act jurisdiction. This section now summarizes key U.S. Supreme Court interpretations from Riverside Bayview Homes, SWANCC, and Rapanos, then provides an overview of both the June 2015 "waters of the United States" rule and the litigation challenging that rule, emphasizing the key issues. Chapter Five now also includes an expanded discussion of stormwater and the two new U.S. Supreme Court cases on Clean Water Act jurisdiction over stormwater.

As was true in the Third Edition, the Fourth Edition continues to increase the number of "The Rest of the Story" notes after cases. These notes trace the context and the aftermath of cases and their continuing implications.