The authors discuss the problem of computing abstraction for hybrid control systems. They propose the notion of abstraction based on the notions of simulation and bisimulation. These notions are presented in a general setting comprising discrete, continuous and hybrid control systems. Several important properties are proved in such general settings, which are directly applicable to hybrid systems. The authors also introduce a composition operator that allows them to construct large-scale, complex hybrid systems by interconnecting smaller hybrid systems. In addition, the authors show that this composition operator is compatible with abstraction and under certain conditions also with bisimulation. According to the authors' comments, several interesting directions for future research remain.