The state of the art calculations of the dielectric function of semiconductors will be reviewed and compared with extant experimental data. Most of the comparisons available in the literature suffer from the fact that the calculations are performed with the atoms at fixed lattice positions, whereas the experimental data are often taken at room temperature. Even when the latter have been obtained at low temperatures, close to 0K, they are affected by the zero-point vibrations, a quantum mechanical effect which can be varied by changing the isotopic masses of the constituent atoms. In recent years samples with different isotopic compositions have become available. Measurements on such samples make possible to determine the renormalization of the dielectric function and other physical properties effected by the lattice vibrations (zero point and temperature effects). This renormalization is particularly large in crystals with light atoms (diamond, ZnO, GaN), a fact that signals a rather strong electron phonon interaction. This strong interaction is responsible for the superconductivity recently observed in boron-doped diamond (also Si and SiC).