There is also another section on conspiracy theories.
http://en.wikipedia.org/wiki/Black_helicoptersBlack Helicopter Facts
The American military community does in fact operate helicopters, particularly the Pave Low which is optimized for long-range stealthy insertion and extraction of personnel, including combat search and rescue.
The US Army regularly conducts both exercises and operational missions in United States airspace. Some of these exercises have taken place in heavily urbanized areas, including Los Angeles, Detroit, San Francisco, Oakland and Washington DC. Most operational missions are tasked in narcotics interdiction in the American Southwest and out of Florida. By extensive use of GPS and night vision equipment, as well as other classified means, are able to fly in zero visibility conditions with no running lights. At this high intensity level of operation, training is necessarily almost as dangerous as actual combat. Frequent practice is necessary to retain proficiency.
At least some sightings of black helicopters are very likely to have been helicopters on exercises and/or missions. This only feeds fuel to the fire for conspiracy theorists who do their homework on military matters. However, from a practical perspective, any extensive missions conducted by the American military on domestic soil would rapidly leak to the news media, and thus to the general public.
Conspiracy theorists would probably not be comforted to know that small-arms fire is almost entirely ineffective against a well-planned helicopter mission, and that the use of automatic weapons would mark the firing positions for counter-fire and air strikes. However, surface to air missiles are particularly effective against helicopters, with many infrared-guided weapons able to acquire targets from any angle of launch.