A driving force for the realization of a sustainable energy supply is the integration of renewable energy resources. Due to their stochastic generation behaviour, energy utilities are confronted with a more complex operation of the underlying power grids. Additionally, due to technological developments, controllable loads, integration with other energy sources, changing regulatory rules, and market liberalization, the system’s operation needs adaptation. Proper operational concepts and intelligent automation provide the basis to turn the existing power system into an intelligent entity, a smart grid. During the last two decades, a growing number of various research, technology development, and innovation activities have already been carried out in this domain. Various demonstration projects have already shown the applicability of the proposed smart grid approaches. Also, the extension of the smart grid concept which focuses on electric energy to multi-domain energy – towards smart energy systems – has been carried out during the last years. While reaping the benefits that come along with those intelligent behaviours, it is expected that system-level design and testing will need to play a significantly larger role in the development and roll-out of future solutions and technologies. Design and validation approaches, concepts, and corresponding tools for smart grids and smart energy systems (also referred to as cyber-physical energy systems) are still not mature enough for effective usage. A key element in this context is the human factor as well as educated professionals, engineers, and researchers understanding the needs and methods for complex smart grids and smart energy systems validation in a multi-domain and cyber-physical manner. This keynote aims to tackle the above-mentioned requirements by highlighting proper concepts, methods, and tools for designing and validating cyber-physical energy systems.