Co-simulation is the cooperative simulation of a system model through different software packages. Collaborative simulation, as such, spans more physics domains and offers more insight than single-domain engines alone. Therefore, the collective composition of its parts enables multi-domain, multi-physics simulation results.ETAP CoSim™ enables ETAP simulation engines to collaborate and interact. For example, the ETAP Time Domain Power Flow can co-simulate with ETAP Harmonic Analysis to assess harmonics distortion over time. ETAP CoSim platform also enables ETAP and 3rd party tools to co-simulate and solve large, complex, and multi-disciplinary system models—collectively via an efficient API surface. The ability to co-simulate with third-party software extends existing software capabilities into the multi-physics domain and greater situational awareness. This solution presentation will introduce the concepts of co-simulation and the flexible ETAP CoSim platform. The presentation will also highlight commercial use-cases of Phasor and Electromagnetic co-simulation using emtCoSim™ and Controller Hardware-in-the-Loop (CHIL).