The use of reversible gas–solid reactions as an energy storage route could offer relevant technological contributions to an energy system predominantly based on renewable energy. However, our current understanding of this technology is mainly based on fundamental material research and generic application concepts. Therefore, the first part of this chapter summarizes the current state of knowledge in order to identify unique advantages that could arise from the utilization of reversible gas–solid reactions. Starting with a technological differentiation between various reversible processes used for energy storage, a classification of different reactor designs and a generic approach for thermal integration and necessary reaction gas supply, three main directions are derived that currently seem most promising for thermochemical energy storage devices: (1) open configurations to reduce system complexity, (2) utilization of available pressure differences to adjust the reaction temperature and (3) combination of abundant materials with the intrinsic possibility of lossless storage. The second part is structured accordingly and summarizes the current status and refers to recent examples.