Abstract The importance of the internal interfaces between structural elements and their surroundings for the electronic properties of nanostructured materials is stressed by giving a brief overview of recent optical experiments on several different kinds of cluster-matter: (1) The Mie-absorption of silver nanoparticles embedded in various solid matrix materials was used to obtain information about static and dynamic charge transfer in the interlayers and their impact on the plasmon dephasing lifetime. (2) Effects of chemical surface reactions are demonstrated for silver clusters reacting with oxygen, sulphur and tripenylphosphine. (3) The occurrence of structural and electronic phase transitions during hydrogenation of yttrium (which is well-known for extended matter) is demonstrated for the first time for clusters. (4) Non-linear optical bi-stability is shown for silver-coated CdS-clusters. This paper was presented at The Fourth International Conference on Nanostructured Materials, NANO ‘98, July 1998, Stockholm/Sweden.