Hurricane Katrina was the costliest natural disaster and one of the five deadliest hurricanes in the history of the United States. The storm is currently ranked as the third most intense United States landfalling tropical cyclone, behind only the 1935 Labor Day hurricane and Hurricane Camille in 1969. Overall, at least 1,245 people died in the hurricane and subsequent floods, making it the deadliest United States hurricane since the 1928 Okeechobee hurricane. Total property damage was estimated at $108 billion (2005 USD), roughly four times the damage wrought by Hurricane Andrew in 1992 in the United States.

The eleventh named storm and fifth hurricane of the 2005 Atlantic hurricane season, Katrina originated over the Bahamas on August 23 from the interaction between a tropical wave and the remnants of Tropical Depression Ten. Early the following day, the new depression intensified into Tropical Storm Katrina. The cyclone headed generally westward toward Florida and strengthened into a hurricane only two hours before making landfall at Hallandale Beach and Aventura on August 25. After very briefly weakening to a tropical storm, Katrina emerged into the Gulf of Mexico on August 26 and began to rapidly deepen. The storm strengthened to a Category 5 hurricane over the warm waters of the Gulf of Mexico, but weakened before making its second landfall as a Category 3 hurricane on August 29 in southeast Louisiana.

Katrina caused severe destruction along the Gulf coast from central Florida to Texas, much of it due to the storm surge and levee failure. Severe property damage occurred in coastal areas, such as Mississippi beachfront towns; over 90 percent of these were flooded. Boats and casino barges rammed buildings, pushing cars and houses inland; water reached 6–12 miles (10–19 km) from the beach.

Over fifty breaches in New Orleans's hurricane surge protection were the cause of the majority of the death and destruction during Katrina on August 29, 2005. Eventually 80% of the city and large tracts of neighboring parishes became flooded, and the floodwaters lingered for weeks. According to a modeling exercise conducted by the U.S. Army Corps of Engineers (USACE), two-thirds of the deaths in Greater New Orleans were due to levee and floodwall failure. All of the major studies concluded that the USACE, the designers and builders of the levee system as mandated by the Flood Control Act of 1965, is responsible. This is mainly due to a decision to use shorter steel sheet pilings in an effort to save money. In January 2008, Judge Stanwood Duval, U.S. District Court, ruled that despite the Corps' role in the flooding, the agency could not be held financially liable because of sovereign immunity in the Flood Control Act of 1928. Exactly ten years after Katrina, J. David Rogers, lead author of a new report in the official journal of the World Water Council concluded that the flooding during Katrina "could have been prevented had the corps retained an external review board to double-check its flood-wall designs."

There was also an investigation of the responses from federal, state and local governments, resulting in the resignation of Federal Emergency Management Agency (FEMA) director Michael D. Brown, and of New Orleans Police Department (NOPD) Superintendent Eddie Compass. Many other government officials were criticized for their responses, especially New Orleans Mayor Ray Nagin, Louisiana Governor Kathleen Blanco, and President George W. Bush. Several agencies including the United States Coast Guard (USCG), National Hurricane Center (NHC), and National Weather Service (NWS) were commended for their actions. They provided accurate hurricane weather tracking forecasts with sufficient lead time.