Within the US Federal Government, the Department of Homeland Security is responsible for preparing for, preventing and responding to domestic emergencies -- particularly terrorist attacks and natural disasters -- that threaten the nation's residents and infrastructure. In order to accomplish this goal, it coordinates and utilizes the efforts of thousands of resources within state, local and federal agencies.

The department was established on November 25, 2002, as a response the the September 11, 2001, terrorist attacks. It consolidated several government offices into one overriding umbrella group. The major components of theDHS are:

• United States Customs and Border Protection (CBP): Protects the nation's borders from terrorists and weapons entering the US, while facilitating the flow of legitimate trade and travel.
• United States Citizenship and Immigration Services: Oversees immigration and naturalization of foreign people entering the country legally.
• United States Immigration and Customs Enforcement (ICE): Identifies vulnerabilities in the nation's border, economic, transportation and infrastructure security.
• United States Coast Guard: Protects America's coasts and any maritime region as required for national security.
• Federal Emergency Management Agency (FEMA): Prepares the country for national incidents, oversees recovery efforts following an incident and administers the National Flood Insurance Program.
• United States Secret Service: Protects the President and other high-level officials and investigates financial crimes like counterfeiting, financial institution fraud, identity theft and computer fraud.
• Transportation Security Administration (TSA): Protects the nation's transportation systems.
• Office of Policy: Provides centralized coordination and planning of departmental practices aimed at protecting the nation.
• Office of Health Affairs: Coordinates departmental medical activities to ensure a proper medical response to incidents.
• Office of Intelligence and Analysis: Uses information and intelligence from multiple sources to identify and assess national threats.
• Office of Operations Coordination: Monitors national security and coordinates activities within the department and with officials from every state and major metropolitan area in the country.
• Federal Law Enforcement Training Center: Provides training to law enforcement professionals.
• Directorate for National Protection and Programs: Works to advance the department's risk-reduction mission, encompassing both physical and virtual (cyber) threats and their associated human elements.
• Directorate for Science and Technology: The primary research and development arm of the DHS, focusing on the department's technological needs.
• Directorate for Management: Responsible for, among other things, DHS budgets and appropriations, human resources, information technology systems, facilities and equipment.
• Domestic Nuclear Detection Office: Aids in the detection and response of federal, state and local governments to nuclear threats.