A key component of modern concentrating solar power plants is a thermal storage unit, which allows adapting the production to the needs by decoupling the energy collection during sunny hours from the power production hours. State of the art is a storage in two tanks via molten nitrate salts, which, however, faces a couple of drawbacks including limited operation temperature (typically 290?565°C), danger of salt freezing, high costs and high CO2 footprint of the salt production. In the SOLAR-ERA.NET project „SLAGSTOCK” a promising alternative thermal storage concept is investigated: the use of one large storage tank filled with a packed bed of metallurgical slag pebbles, through which the heat transfer fluid (molten salt or air) flows and which is heated (charged) during the sunny hours and is cooled (discharged) as needed to generate electricity. Six partners from four countries have investigated the major aspects of the new process including the realisation of a 1 m3 test tank for storing about 350 kWhth from hot air. The global steel manufacturer ArcelorMittal provided the metallurgical slags, which were processed by two alternative routes to few cm sized slag pebbles suited for the packed bed. In parallel studies of the relevant physical properties of the pebbles were conducted, the compatibility of the slag with molten salt and hot air was investigated and the behaviour of industrial sized storage tanks (for 1 GWhth) was modelled, considering charging and discharging cycles. The specific work of the Swiss partner PSI consisted in the modelling of the storage tank with molten salt as heat transfer fluid and in the performance of a life cycle assessment (LCA) for the new process, clearly demonstrating the advantages of the new process compared to the established 2-tank molten salt process. Depending on the detailed process variant the equivalent CO2-emissions can be approximately reduced to one half. The results of „SLAGSTOCK” are amongst others contributing to the related large EU-project „RESLAG”, from which the further development of the innovative slag based thermal storage process might take place.