While ground-state mixed-valence systems are very well known, excited-state mixed-valence (ES-MV) systems received comparably less attention. Intervalence charge-transfer (IVCT) excited states, locally excited mixed-valence (LE-MV) systems, and photoinduced mixed-valence (PI-MV) systems are different types of ES-MV systems. They are the simplest models for excited-state electron transfer reactions, and therefore crucial in the development of solar energy conversion schemes. IVCT exited states are the best-known ES-MV systems and are usually called electronic isomers or redox isomers of the ground state. Their back electron transfer reactivity depends on whether it proceeds with or without spin conservation. LE-MV systems feature locally excited donors or acceptors, and their reactivity might be dominated by locally excited states or IVCT excited states. PI-MV systems include charge transfer counterparts, which can adopt different spatial distributions with respect to the mixed-valence core. Depending on the location of the charge-transfer counterpart, drastic changes in the electronic configuration of PI-MV systems can arise, with equally important impact on their reactivity in comparison to only the donor or the acceptor. This review addresses different processes that lead to the generation of ES-MV systems, their properties, reactivity, and spectroscopy. Additionally, the experimental techniques usually employed to study ES-MV systems are briefly discussed, together with theoretical formalisms and classifications which were developed for the ground state but can also be applied to the excited state.