This document summarizes the history of regulatory definitions and court rulings related to "waters of the United States" (WOTUS) under the Clean Water Act. It discusses key Supreme Court cases in 2001 and 2006 that narrowed the scope of federal jurisdiction over wetlands and isolated waters. Subsequently, the Obama and Trump administrations issued new rules expanding and contracting the definition of WOTUS. The Biden administration is now reviewing the Trump-era rule and plans to establish a new durable definition through public input and regional roundtables.