[citation needed] In the 20th century, unlike United States, Europe did not have a single civilian airspace – i.e., one in which air navigation is managed at the European level.
Furthermore, European airspace is among the busiest in the world with over 33,000 flights daily and airport density in Europe is very high.
The EU Single European Sky (SES) legislature was drawn to overcome the air control management's fragmentation and flight capacity limitation by structuring airspace and air navigation services at a pan-European level.
To develop the needed technological capacity, the SESAR program was initiated in 2004 as a continuation of a smaller effort by the Eurocontrol SESAME project.
In June 2010, European and American authorities reached a preliminary agreement on interoperability between their future air traffic management systems, SESAR and NextGen.