## 1 Introduction  
Europe's electricity system is undergoing a profound transformation driven by two overarching imperatives: decarbonization and digitalization [1]. As renewable energy sources proliferate and the electrification of transport and heating accelerates, power grids must become smarter, more responsive, and more efficient to accommodate fluctuating generation and increasing demand [2]. In this context, the concept of a digital twin, a sophisticated virtual model of the physical electricity grid that remains continuously synchronized with real-world data, has emerged as a key enabler [3]. While the term \"digital twin\" is used broadly across sectors, in the context of power systems, it can be differentiated into three main types: (1) asset-level twins, which simulate the condition and behavior of specific grid components such as transformers or substations; (2) system-level twins, which represent the grid topology, flows, and operational constraints in real time; and (3) multi-domain or federated twins, which integrate grid data with inputs from other infrastructures such as weather, transport, or markets. This review focuses on the system-level and federated digital twins that underpin large-scale operational and planning capabilities across the European electricity grid. These layers are central to enabling the cross-border coordination and flexibility required by the EU's energy transition vision [4; 5]. By mirroring the state of assets and grid conditions in real time, a digital twin can support predictive analysis, scenario simulation, and data-driven decision-making [4; 6]. Recognizing this potential, the European Union (EU) has placed the development of a continental-scale digital twin of the electricity grid at the forefront of its policy agenda [1]. In October 2022, the European Commission released its \"Digitalizing the Energy System--EU Action Plan\" [7], explicitly calling for a secure and federated digital twin to improve grid intelligence, drive innovation, and support real-time system operations [1]. In December 2022, Europe's transmission system operators (TSOs), represented by ENTSO-E, and distribution system operators (DSOs), under the EU DSO Entity, signed a joint declaration of intent to collaboratively advance this vision [8; 9], an initiative that was welcomed by the European Commission as instrumental for enhancing flexibility, reliability, and coordinated planning across member states [10]. Several large-scale projects have emerged in response to these mandates. Central among them is the Horizon Europe-funded TwinEU consortium, which unites 75 partners from over a dozen EU countries [11]. TwinEU adopts a \"federated\" or \"system-of-systems\" approach, linking local digital twins operated by TSOs, DSOs, and technology providers into a unified platform spanning high-voltage transmission networks down to low-voltage grids [3]. By demonstrating applications, such as congestion management, predictive maintenance, and renewable energy integration, at eight pilot sites across eleven countries, TwinEU aims to validate how modular sub-twins can collectively represent Europe's interconnected power system [5; 12]. This aligns with the broader collaborative ethos in the EU electricity sector, where multiple operators and regulatory bodies across 27 member states must cooperate. Beyond these targeted initiatives, the EU is laying critical groundwork in data policy and high-performance computing that directly supports digital twin development. The Common European Energy Data Space (CEEDS), created under the EU Digital Strategy, provides a framework for interoperable and secure data exchange [13]. Meanwhile, the Destination Earth (DestinE) program, focused on building a high-fidelity digital model of the planet, could eventually enable multi-domain integration by linking electricity system models with climate and mobility datasets [14].