Congress is more ideologically polarized than at any time in the modern regulatory era, which makes legislation ever harder to pass. As a result, Congress is increasingly absent from the policymaking process, and fails to regularly update statutes in the face of social, economic and technological change. This leaves agencies to adapt old statutes to new problems. The challenge of managing statutory obsolescence affects many agencies, and arises in areas as diverse as financial, telecommunications, and food and drug regulation. We examine this dynamic in two fast-moving policy domains, environmental and energy regulation, where Congress has been remarkably absent in recent decades. Contrary to what some might suspect, we find that agencies m...