Congestion has plagued air traffic in the US and in Europe for nearly 20 years. To protect air traffic control from overloads, air traffic flow management tries to anticipate and prevent overloads and limit resulting delays. This paper focuses on understanding the requirements for developing re-routing decision support systems (DSS). It identifies participants in re-routing decisions and investigates the concept and need for a re-routing decision support system. A re-routing demonstrator is discussed as a first step in the development of a DSS and a demonstrator for pre-tactical and tactical re-routings is described. User feedback is described and issues of automation and complexity of re-routing DSS discussed. Finally, the integration of re-routing DSS in future air traffic management systems is addressed.