In contrast to fossil fuel or nuclear energy based electrical power intermittent renewable energy such as solar and wind need to balance the mismatch of energy supply and demand to allow stable and secure grid operation requiring energy storage technologies. A promising device is the solid oxide electrochemical cell (SOC) which can be operated reversibly, i.e. an SOC can act as an electrolyser to store electricity in the form of hydrogen and it can act as a fuel cell to produce electricity, water and heat. With this technology a single stack can be integrated into one system to address different markets such as hydrogen production, power-to-gas, energy storage and distributed power generation. When electrolyzing both steam and CO2 in co-electrolysis operating mode, synthesis gas can be produced to be converted by further downstream catalytic processes into fuels such as methane, gasoline or diesel. Due to the high operating temperature of 750-900 °C and the possibility to re-use waste heat from industrial processes very high electrical efficiency can be achieved. However, maintaining the performance during long-term operation represents still a major challenge. Solid oxide cells and stacks are characterized and tested at DLR regarding electrochemical performance and degradation for both reversible and co-electrolysis operation. In cooperation with a car manufacturer (AUDI AG, Germany) and a stack supplier (Sunfire GmbH, Germany) DLR works on the investigation of SOC stacks during near-system operating conditions in electrolysis as well as reversible operating mode. The electrochemical performance is monitored during long-term tests to be compared with identical stacks implemented in an industrial reversible SOC (RSOC) system in order to determine and better understand degradation processes occurring in different operating modes. The concept of the power-to-gas facility with 300 kW power and first results of stack tests are presented. DLR also works on the evaluation of solid oxide cells for co-electrolysis of CO2 and H2O aiming at the production of synthetic fuels. Cell behaviour and durability are assessed under various operating conditions. Results will be reported and discussed and remaining challenges for maturing the technology are addressed.