The US Department of Defense influenza surveillance system now spans nearly 500 sites in 75 countries, including active duty US military and dependent populations as well as host-country civilian and military personnel. This system represents a major part of the US Government's contributions to the World Health Organization's Global Influenza Surveillance Network and addresses Presidential Directive NSTC-7 to expand global surveillance, training, research and response to emerging infectious disease threats. Since 2006, the system has expanded significantly in response to rising pandemic influenza concerns. The expanded system has played a critical role in the detection and monitoring of ongoing H5N1 outbreaks worldwide as well as in the initial detection of, and response to, the current (H1N1) 2009 influenza pandemic. This article describes the system, details its contributions and the critical gaps that it is filling, and discusses future plans.