Ever since discrete event simulation has been adopted by a large research community, simulation developers have attempted to draw benefits from executing a simulation on multiple processing units in parallel. Hence, a wide range of research has been conducted on Parallel Discrete Event Simulation (PDES). In this chapter we give an overview of the challenges and approaches of parallel simulation. Furthermore, we present a survey of the parallelization capabilities of the network simulators OMNeT++, ns-2, DSIM and JiST.