The Air Traffic Control system of a country manages all the aircrafts that fly in its airspace, designs control sectors, manages the flows between the different airports and beacons, ensures separation between aircraft during their flight, take off and landing. Thus, it operates at different levels, each one of them designed to provide control, ensure safety, and limit the traffic passed to the following level. In this paper, we show how Genetic Algorithms can improve some of the tasks manually done by the ATC system. After a brief description of GAs, some of the improvements used (simulated annealing, sharing), we study three applications of GAs to ATC. We first show an application of GAs to en-route conflict resolution. Then we give an example on GAs used to optimize air space sectoring. The last part gives an application of GAs to traffic assignment.