Spin-crossover (SCO) systems are molecules or materials that can switch between two alternative spin-states, thus exhibiting switching behaviour. The transition from the low-to the high-spin state can be triggered using an external stimulus, commonly temperature, and with the spin-state change there are profound changes in the physical properties of the system. This duality is very appealing from the technological point of view, because one can envision harvesting such materials for molecular level devices or spintronic applications. The SCO phenomenon, firstly reported nearly a century ago, has grown at the interface between physics and chemistry, and the number of compounds exhibiting SCO behaviour has vastly increased over the last years.