Chemisorption is a promising solution for long-term heat storage with a high energy density. It is thus suitable for interseasonal heat storage. Considering applications to buildings, heat storage can be used for space heating and/or domestic hot water production. In this chapter, the specifications for a heat storage system for buildings are given. A review of chemical heat storage materials is presented and reactor development is discussed. The technology readiness level of such storage systems is low; some future research topics are presented in the conclusions.