Recently, nanoscale spin-crossover (SCO) particles have been the subject of great interest. The change in the 3d electronic configuration of the metal ion results in significant changes in the metal-ligand bond length and geometry, as well as in the molecular volume. Hence the spin switching process is accompanied by a remarkable change in the color, mechanical properties, dielectric properties, and magnetic susceptibility. The synthesis and investigation of these materials at reduced length scales is central not only to the exploration of fundamental effects of size reduction in these systems, but also for the development of new functional materials with applications, including guest molecule sensing, memory devices, and molecular switches