An introduction to the stress-induced birefringence of solids, with emphasis on cubic and amorphous materials, is given. Most available experimental data have been obtained in the frequency region below the electronic absorption edge : the corresponding coefficients of the stress-optical tensor are then real. Above the edge (and also in the IR region of the Reststrahlen) they become complex. Ellipsometry is an excellent tool for the investigation of complex stress-optical functions. It also yields the hydrostatic pressure induced changes in the dielectric functions. Data obtained recently for diamond and zincblende-type crystals and their theoretical interpretation are discussed.