This chapter provides a comprehensive account on the state of art of glass nanospheres and artificial opals by describing the mechanisms of growth for the synthesis of monodisperse dielectric nanospheres and the conditions for the realization of colloidal crystals. It describes the mechanism of growth and aims to report the conditions necessary for the realization of monodisperse particles of different materials such as polystyrene, silica, silica activated with rare earth ions, and gold-silica core–shell particles. Among the different structures, gold-silica core–shell particles have attracted a wide interest not only as probes but also as building blocks for the creation of hybrid nano-objects. The chapter focuses on the conditions for the realization of colloidal crystals and describe the main structural and optical features, putting in evidence the characterization approaches for their determination. It discusses novel applications of opals related to the modification of the spontaneous emission, and to the implementation of physical, chemical, and biological sensors.