We present GaliLEO, a simulator for the transmission of connection-oriented traffic over a constellation of LEO/MEO (Low / Medium Earth Orbit) satellites. Its scope is limited to the satellites and the stations accessing them, without any modelling of the terrestrial network, but inside this scope the goal is to study the performance of satellitebased communication networks from as many as possible points of view at the network level. Typical applications include simulation of access techniques, routing policies, fault management. The simulator is written in Java, and it makes use of dynamic loading to easily integrate userwritten modules. A draft manual is available, and a preliminary version of the program has been published at the end of 2000.