This contribution reports on the state of the art of the elaboration and the application of nanoparticles (NPs) and nanohybrid/nanocomposite materials based on spin-crossover (SCO) complexes. The first part of this review concerns the syntheses and the characterizations of the physical properties of SCO NPs. All of the methods including homogeneous and heterogeneous media syntheses developed for the elaboration of such NPs and the associated methods used for their morphological characterization are presented. A particular attention is paid on the effects of the size reduction and the influence of the environment on the SCO properties and to specific and recent remarkable advanced physical measurements realized on a batch of NPs or on an isolated object. The second part presents the elaboration of various nanocomposite or nanohybrid materials for which SCO NPs have been associated with magnetic entities, noble metals, different fluorescent dyes, and different active polymers with the objectives to go toward specific applications based on synergistic effects between the two components.