As of 2003, the United States Secret Service is a law enforcement agency under Homeland Security. Formerly, as with a lot of services folded in Homeland Security, it was a Treasury department. The Secret Service is responsible for preventing counterfeit, theft and major fraud, but more famously, it protects U.S. leaders and their families, as well as visiting heads of state and executive candidates.
The United States' first true domestic intelligence and counterintelligence agency, the Secret Service eventually saw many of its responsibilities handed off to the FBI, ATF, ICE and IRS. It gained its protective authority in 1901, after the assassination of President William McKinley. While former presidents and vice-presidents once earned the protection of the Secret Service for life, 1997 legislation limited that service period to 10 years, moving forward.
The Secret Service Uniformed Division (UD) was established in 1922 as the White House Police Force before being integrated into the Secret Service in 1930. In 2010, the UD numbered more than 1300, providing security for the White House complex, the vice-president's residence, the Treasury and diplomats in the District of Columbia. They also provide officers for countersniper support, the K-9 and explosives unit and the emergency response team.
The Presidential Advance Team -- known as the most complex, expensive and thorough advance unit in the world -- includes logistics and security for the president's motorcade, as well as employing Secret Service against the 500 death threats the president receives each month, on average.
Protecting the president is no small affair and takes a great deal of manpower. For example, during President Bush's administration in 2003, a trip to London required 904 staffers from Defense, 600 from the armed services, 250 Secret Service officers, 205 White House staff, 103 CIA staff, 44 staff from the State Department, 30 more from within the Cabinet, 18 Senior Advance Office staff, 16 Congressmen and 12 sniffer dogs.