Real-time network simulation refers to simulating computer networks in real time so that the virtual network can interact with real implementations of network protocols, network services, and??distributed applications. In this paper, we present the motivation behind real-time network simulation and compare it against other major networking research tools, including analytical models, physical testbeds, simulation, and emulation. We introduce PRIME, a??parallel real-time network simulator, and provide a summary of techniques that allow PRIME to model large-scale networks and interact with many real applications under the real-time constraint. We also discuss ongoing research efforts that will allow real-time network simulation to assume important roles at supporting future networking research.