Cluster compounds represent an important class of molecules that lie between the bulk metal and conventional chemistry based on single metal centres. They contain three to 150 metal atoms bonded together by covalent bonds in a polyhedral arrangement that does not necessarily conform to that found in the bulk metal. Their electronic structure has been interpreted using free-electron spherical harmonic models that relate the polyhedral features to the total number of valence electrons in the molecule. The occurrence of these molecules raises interesting questions regarding the transition from metallic to non-metallic properties.