In modern power systems high penetration of renewable energy sources and decentralized paradigm are regarded as the path toward more sustainable electricity landscape. This includes distributed energy resources whose intermittency and uncertainty may cause issues to the system operators. Distribution system operators have an obligation to ensure secure and stable system operation. Hence, they seek the most efficient methods to deal with these challenges. Flexibility procurement is considered as one of the prerequisites for painless and successful integration of renewable sources. Furthermore, distribution-level flexibility markets are modeled and tested to trade flexibility locally, solve congestion issues and defer grid expansion. This paper surveys the ongoing research in the field of flexibility markets, its design, open questions and most promising research projects. The key stakeholders are identified, overview of the current trends in the power system and research initiatives are presented, accompanied with the dilemmas being discussed in the power systems community.

The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 863876 in the context of the FLEXGRID project. The contents of this document are the sole responsibility of authors and can under no circumstances be regarded as reflecting the position of the European Union.

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The FLEXGRID project develops a digital platform designed to offer Digital Energy Services (DESs) that facilitate energy sector stakeholders (i.e. Distribution System Operators - DSOs, Transmission System Operators - TSOs, market operators, Renewable Energy Sources - RES producers, retailers, flexibility aggregators) towards: i) automating and optimizing the planning and operation/management of their systems/assets, and ii) interacting in a dynamic and efficient way with their environment (electricity system) and the rest of the stakeholders. In this way, FLEXGRID envisages secure, sustainable, competitive, and affordable smart grids. A key objective is the incentivization of large-scale bottom-up investments in Distributed Energy Resources (DERs) through innovative smart grid management. Towards this goal, FLEXGRID develops innovative data models and energy market architectures (with high liquidity and efficiency) that effectively manage smart grids through an advanced TSO-DSO interaction as well as interactions between Transmission Network and Distribution Network level energy markets. Consequently, and through intelligence that exploits the innovation of the proposed market architecture, FLEXGRID develops investment tools able to examine in depth the emerging energy ecosystem and allow in this way: i) the financial sustainability of DER investors, and ii) the market liquidity/efficiency through advanced exploitation of DERs and intelligent network upgrades.