Colloids have long been used in applications such as paints, coatings, foods, and many manufacturing processes. Recently, synthetic crystalline arrays of colloidal particles have been used as novel optical materials such as diffractive filters, mimicking the optical properties of opals—natural colloidal crystals made from silica spheres. Colloidal assembly has been proposed to manufacture photonic bandgap materials that can be tailored and that could have many uses in optical devices. The advantages of using colloids to do the self-assembly of novel materials are the relative ease with which monodisperse spheres comparable in size to the wavelength of light can be manufactured and also the demonstrated ease by which some suspensions of monodisperse colloidal spheres crystallize when placed under favorable conditions. Before we can use colloidal crystallization as a controlled self-assembly technique for making novel optical materials, we need (1) to create a means of manufacturing large quantities of monodisperse particles of the desired dielectric behavior, (2) to understand the phase diagram and nucleation phenomena of colloidal suspensions, and (3) to find an easy means to fix the particles in place once they selforganize. In this article, I focus on the second point just mentioned, I give an overview of the phases and some of the complex phenomena encountered in three-dimensional (3D) suspensions and in thin layers of monodisperse colloidal spheres between smooth walls, and I then briefly mention the greater complexity encountered in bidisperse systems. The first and third points will be dealt with elsewhere in this issue.