This review deals with the preparation, properties, and interactions of well-defined colloids of simple and composite nature. The formation of such dispersions by precipitation from homogeneous solutions is described and some problems related to the mechanisms of the formation and growth of "monodispersed" particles are cited. A few examples of the use of uniform dispersions in the evaluation of their properties as a function of particle size and shape are offered, with specific reference to optical and magnetic phenomena. Finally, some experimental results in (hetero)coagulation and particle adhesion processes are described, as obtained with well-defined colloids, in order to evaluate the theories of interactions in systems consisting of dissimilar particles.