Virtual system development (“frontloading”) is getting more and more important in a plenitude of industrial domains to reduce development times, stranded costs and time-to-market. Co-simulation is a particularly promising approach for interoperable modular development. However, the coupling and integration of real-time systems into simulation environments (especially of systems of distributed HiL systems and simulations) still requires enormous effort. The aim of ACOSAR is to develop both a non-proprietary “Advanced Co-simulation Interface” (ACI) for RT-System integration and an according integration methodology, which shall be a substantial contribution to international standardization (FMI). The results of ACOSAR will lead to a modular, considerably more flexible as well as shorter system development process for numerous industrial domains and will enable the establishment of new business models.