We consider some of the factors which affect the magnitudes of the tensor coefficients describing the nonlinear optical effects of second-harmonic generation and the Pockels or linear electro-optic effect. The effects of molecular geometry are our particular concern. The electro-optic coefficients of double sulphates with the langbeinite structure are compared with those of tetragonal phosphates and arsenates by ascribing the nonlinearitiee to the XO₄ tetrahedra. The unfavourable orientation of the SO₄ groups is shown to be responsible for the low electro-optic coefficients of the langbeinites. These ideas are carried a stage further by assigning the nonlinearities to individual bonds. By this means the signs and magnitudes of the second-harmonic generation coefficients of several niobates are predicted from those of lithium niobate. Similar calculations are carried out for iodates, and empirical nonlinear optical bond polarisabilities are evaluated for a number of complex materials containing covalently bonded oxygen atoms. It is shown that the nonlinear polarisability depends on the electronegativity difference between the atoms forming the bond, and on their coordination. A further chapter uses the well-known anharmonic oscillator model to establish a connection between nonlinear optical effects and optical activity in crystals of sodium chlorate and sodium bromate.