The contribution of the valence electrons to the nonlinear optical susceptibilities may be estimated on the basis of simple tetrahedral bonding orbitals. The coefficient for second-harmonic generation in III-V compounds is in satisfactory agreement with the theoretical estimate. The same model contributes to the third-order nonlinearity, which describes scattering processes between four light waves. Again reasonable agreement is obtained with data for combination frequency generation in Si and Ge. The much larger effects in n-type InAs and InSb are due to conduction electrons. Their contribution has been calculated exactly under the assumption of Kane's theory for the band structure. The contribution of the valence electrons is, however, not negligible.