Coreso is the Regional Coordination Service Centre subsidiary of 50Hertz Transmission (Germany), EirGrid (Ireland), Elia (Belgium), National Grid ESO (United Kingdom), REE (Spain), REN (Portugal), RTE (France), Terna (Italy) and SONI (Northern Ireland). The area monitored by Coreso's Brussels-based technical coordination centre covers 275 million customers, some 54% of the European Union’s population. Coreso started its operations in February 2009 and is the first joint technical coordination centre owned by several transmission system operators. Coreso has delivered substantial improvements in operational coordination between transmission grids in response to new challenges. The growth of renewable energy sources – characterized by their variable output - coupled with the rise in cross-border exchanges in the European electricity market, are leading to increasing fluctuation in power flows. Coreso has developed a high degree of reliability and expertise. On several occasions, Coreso demonstrated its value by identifying in the electrical system risk situations that are only detectable by an overall sharing, beyond the national perimeter of each individual transmission network. In practice, Coreso provides the control centres of participating TSOs with detailed forecasts of network security in this extended coverage zone, by producing security analyses, simulating a range of different scenarios and suggesting coordinated measures. Coreso coordinates exchanges between the national control centres, which remain operationally responsible on their respective networks.