Colloid chemistry is the science of the finely divided state of material. Colloids are small units of gas, a liquid, or a solid whose size is measured in nanometers. Systems containing or made up of colloids can have special properties differing significantly from those of the molecules from which the colloid particles are made and from those of the corresponding bulk material. The colloidal state is therefore sometimes described as the "fourth state of aggregation". Fundamental properties such as melting point, conductivity, or the optical parameters depend upon particle size. Modem colloid chemistry thus provides considerable scope for the production of new materials with new combinations of properties through control of the chemical composition on a nanometer scale, These possibilities are considered in the light of a number of research projects being pursued at the Max Planck Institute for Collid and Interface Research.