The events of September 11, 2001, led to profound changes in government policies and structures to confront homeland security threats. Most notably, the Department of Homeland Security (DHS) began operations in 2003 with key missions that included preventing terrorist attacks from occurring in the United States, and minimizing the damages from any attacks that may occur. DHS is now the third-largest federal department, with more than 200,000 employees and an annual budget of more than $50 billion. Since 2003, GAO has issued over 1,000 products on DHS's operations in such areas as border and transportation security and emergency management, among others. As requested, this report addresses DHS's progress in implementing its homeland security missions since it began operations, work remaining, and issues affecting implementation efforts. This report is based on GAO's past and ongoing work, supplemented with DHS Office of Inspector General reports, with an emphasis on reports issued since 2008. GAO also analyzed information provided by DHS in July and August 2011 on recent actions taken in response to prior work.