From the NORADSanta.org:
The NORAD Tracks Santa (NTS) program has been around for a long time, since 1955 to be exact! We believe that Santa Claus is alive and well in the hearts of people throughout the world.
NORAD uses four high-tech systems to track Santa - radar, satellites, Santa Cams and jet fighter aircraft.
Detecting Santa all starts with the NORAD radar system called the North Warning System. This powerful radar system has 47 installations strung across the northern border of North America. NORAD makes a point of checking the radar closely for indications of Santa Claus leaving the North Pole on Christmas Eve.
Santa can also be tracked on Twitter!