Some molecular photoisomers can be isomerized to a metastable high-energy state by exposure to light. These molecules can then be thermally or catalytically converted back to their initial state, releasing heat in the process. Such a reversible photochemical process has been considered for developing molecular solar thermal (MOST) systems. In this review, we introduce the concept, criteria, and state-of-the-art of MOST systems, with an emphasis on the three most promising molecular systems: norbornadiene/quadricyclane, E/Z-azobenzene, and dihydroazulene/vinylheptafulvene. After discussing the fundamental working principles, we focus on molecular design strategies for improving solar energy storage performance, remaining challenges, and potential focus areas. Finally, we summarize the current molecular incorporation into functional devices and conclude with a perspective on challenges and future directions. Z.W. and K.M.-P. would like to thank the financial support from K. & A. Wallenberg foundation, the Swedish Foundation for Strategic Research, the Swedish research foundation FORMAS, and the European Union’s Horizon 2020 research and innovation program under grant agreement No. 951801. K.M.-P. acknowledges support from the Catalan Institution for Research and Advanced Studies. P.E. thanks the K. & A. Wallenberg foundation (2014.0226) and the Swedish Research Council (2020-04935). T.L. and Z.-Y.Z. thank the financial support from National Key Research and Development Program of China (2017YFA0207500), National Natural Science Foundation of China (22022507, 51973111), and Beijing National Laboratory for Molecular Sciences (BNLMS202004). D.S. thanks the financial support from MINECO/FEDER (CTQ2017-87372-P). O.B., J.L., and H.A.W. thank the Deutsche Forschungsgemeinschaft (DFG, project 392607742, 62201757). Additional support by the DFG is acknowledged through the Research Unit FOR 1878 “funCOS – Functional Molecular Structures on Complex Oxide Surfaces.” J.L. and O.B. acknowledge the cooperation with the groups of Julien Bachmann, Christian Papp, Andreas Hirsch, Andreas Görling, and Hans-Peter Steinrück in the framework of the studies described in this review. M.B.N. thanks the University of Copenhagen for financial support. With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S). Peer reviewed