Hurricane Katrina of the 2005 Atlantic hurricane season was the costliest hurricane, as well as one of the five deadliest, in the history of the United States. Among recorded Atlantic hurricanes, it was the sixth strongest overall. Hurricane Katrina formed over the Bahamas on August 23, 2005, and crossed southern Florida as a moderate Category 1 hurricane, causing some deaths and flooding there before strengthening rapidly in the Gulf of Mexico. The storm weakened before making its second landfall as a Category 3 storm on the morning of Monday, August 29 in southeast Louisiana. It caused severe destruction along the Gulf coast from central Florida to Texas, much of it due to the storm surge. The most severe loss of life and property damage occurred in New Orleans, Louisiana, which flooded as the levee system catastrophically failed, in many cases hours after the storm had moved inland. The federal flood protection system in New Orleans failed at more than fifty places. Nearly every levee in metro New Orleans was breached as Hurricane Katrina passed just east of the city limits. Eventually 80% of the city became flooded and also large tracts of neighboring parishes, and the floodwaters lingered for weeks. At least 1,836 people lost their lives in the actual hurricane and in the subsequent floods, making it the deadliest U.S. hurricane since the 1928 Okeechobee Hurricane. The storm is estimated to have been responsible for $81.2 billion (2005 U.S. dollars) in damage, making it the costliest tropical cyclone in U.S. history.

The levee failures prompted investigations of their design and construction which belongs solely to the US Army Corps of Engineers as mandated in the Flood Control Act of 1965. There was also an investigation of the responses from federal, state and local governments, resulting in the resignation of Federal Emergency Management Agency director Michael D. Brown. Conversely, the National Hurricane Center and National Weather Service were widely commended for accurate forecasts and abundant lead time. Three years later, thousands of displaced residents in Mississippi and Louisiana were still living in trailers.