Electronic structure becomes of interest when orbitals of neighbouring atoms overlap sufficiently to form bands. This is the case in metals and semiconductors where valence electrons are delocalised and explore the entire size of the nanostructure, which leads to quantum size effects. The band gap in semiconductors and the density of states within the bands become size-dependent. This affects metallic conductivity and colour, i.e. the wavelengths of absorbed and emitted light. Most importantly, the energies of the highest occupied and the lowest unoccupied states change by up to several eV, which affects the availability of electrons for redox reactions and bond formation. Size therefore has an enormous impact on the chemical properties of clusters and (catalytic) nanoparticles. Moreover, the delocalised molecular orbitals of spherical clusters resemble atomic orbitals. It is therefore fruitful for the understanding of properties to regard such clusters as superatoms.