Modular, intelligent and flexible production plants are a promising approach to manage an ever more volatile market with higher flexibility needs and shorter product lifecycles. To guarantee safety and functionality of a plant during operation, the method of virtual commissioning allows testing and validation of automation and control systems before operating the real plant. In this contribution we introduce processes and an environment that intrinsically supports simulation, emulation and validation in the context of modular plants. Standardization of interfaces for virtual models and the technology of co-simulation plays a major role in the concept.