DHS is responsible for protecting the United States from terrorism, securing the nation's borders, and coordinating the federal response to natural disasters. Formed by merging 22 previously separate federal agencies, it is the largest cabinet reorganization since the Defense Department's creation in 1947.

The department's components include Customs and Border Protection, which secures the border and staffs official ports of entry, Immigration and Customs Enforcement, which handles interior immigration enforcement, the Transportation Security Administration, which screens airline passengers, and the Federal Emergency Management Agency, which coordinates disaster response.

DHS also houses the Secret Service, which protects the president and other senior officials, the Coast Guard, which normally operates under DHS but shifts to Defense Department control during wartime, and the Cybersecurity and Infrastructure Security Agency, which defends federal networks and critical infrastructure against cyberattacks.

DHS is the third-largest Cabinet department by staff, and its components long predate it: the Coast Guard traces to 1790, the Secret Service to 1865, and the Customs Service to the First Congress. It also includes U.S. Citizenship and Immigration Services (which processes green cards and naturalization on application fees), the Federal Law Enforcement Training Centers, and a large intelligence and analysis office. Its component agencies have repeatedly ranked near the bottom of federal employee-satisfaction surveys, a legacy of the rushed 2003 merger.