In aviation, a non-towered airport is an airport without a control tower, or air traffic control (ATC) unit. The vast majority of the world's airports are non-towered.[citation needed] In the United States, there are close to 20,000 non-towered airports compared to approximately 500 airports with control towers.[1] Airports with a control tower without 24/7 ATC service follow non-towered airport procedures when the tower is closed but the airport remains open, for example at night.[2]