Publisher Summary This chapter deals with the application of monodispersed particles to fundamental studies. Charged colloidal suspensions form an ordered structure called colloidal crystals when the extraneous electrolytes are removed by ion-exchange resins. If the volume fraction of the colloidal particles becomes high, or the electrolyte concentration is at an intermediate level, one can observe a phase separation of ordered and disordered phases, or crystal and liquid or gas phases. For the study of these phenomena, monodispersed polystyrene spheres prepared by the soap-free emulsion polymerization and silica spheres prepared by hydrolysis of silicon alkoxides are widely used because it is relatively easy to obtain well-defined monodisperse spherical particles with a variety of sizes and surface charges. Furthermore, the optoelectronic properties of fine particles are quite sensitive to particle size, crystal habit, and composition. Hence, for the precise analysis of these properties, monodispersed particles are used. Finally, the ionic and electronic properties of fine particles are strongly dependent on their size, crystal habit, and composition. Hence, for studies of these properties, it is necessary to use monodisperse and well-controlled particles in size, shape, and composition.