A flu pandemic is a worldwide epidemic of an influenza virus. As such, the United States response to a flu pandemic would have both international and domestic components. Additionally, the domestic response effort would include contributions from every governmental level (local, state, tribal, and federal), non-governmental organizations, and the private sector. This report will focus largely on the role of the Department of Defense (DOD) in supporting the nations domestic response effort, although it will also touch on DODs international role. The Department of State would lead the federal governments international response efforts, while the Department of Homeland Security and the Department of Health and Human Services would lead the federal governments domestic response. The Department of Defense would likely be called upon to support both the international and domestic efforts. An analysis of the tasks assigned by the National Strategy for Pandemic Influenza Implementation Plan indicates that DODs role during a flu pandemic would center on the following objectives: assisting in disease surveillance; assisting partner nations, particularly through military-to-military assistance; protecting and treating US forces and dependents; and providing support to civil authorities in the United States With respect to providing support to civil authorities in the United States, the types of defense support which would likely be in greatest demand during a flu pandemic include: providing disease surveillance and laboratory diagnostics; transporting response teams, vaccines, medical equipment, supplies, diagnostic devices, pharmaceuticals and blood products; treating patients; evacuating the ill and injured; processing and tracking patients; providing base and installation support to federal, state, local, and tribal agencies; controlling movement into and out of areas, or across borders, with affected populations; supporting law enforcement; supporting quarantine enforcement; restoring damaged public utilities; and providing mortuary services. Note, however, that DODs ability to support these requests would be limited by its national defense and force protection responsibilities. The two principal ways in which defense support could be provided to civil authorities are by way of an immediate response, or in response to a formal request for assistance (RFA). Additionally, in extreme circumstances the federal government may expedite or suspend the RFA process and initiate a proactive federal response. National Guard personnel would almost certainly be involved in domestic response efforts as members of their state militia under the control of their governor. Current DOD plans do not anticipate federal mobilization of the National Guard or Reserves to respond to a flu pandemic. However, these plans could be modified if circumstances warranted it (for example, if the severity of the pandemic significantly exceeded DODs planning assumptions). In the event such a federal mobilization is contemplated, an important consideration would be the impact it would have on any response efforts that were already occurring at the state and local levels. For example, the activation of Reserve and National Guard medical personnel may pull them out of local hospitals where they are already engaged in the response effort, thereby undermining state and local response efforts.