The ambitious goals of transition towards smart energy systems should be accomplished in a cost-effective way that does not violate power system limits and constraints but keeps the voltage and frequency within limits while satisfying each consumer’s need. To achieve that, significant changes in the current system operation are required in terms of policy changing, low-carbon technologies implementation, data collection and management and communication protocols. Moreover, implementation of different approaches in both transmission grid (TG) and distribution grid (DG) management will be achieved through development of various tools for long-term and day-ahead planning, real-time operation and control together with asset management tools considering energy consumption and generation profiles, ancillary services schedules, smart grid technology, such as on-load tap changing (OLTC) transformers, energy storage devices, controllable distributed generation, flexible loads and electric vehicles. The presentation will introduce the existing/known TSO-DSO coordination mechanisms and present a novel, proposed coordination scheme envisioned within the Horizon 2020 project ATTEST. It will discuss ATTEST developed operational tools both on the transmission and distribution side and present simulation and demonstration environment where the presented tools and mechanisms will be tested.