A theory of second-harmonic generation in spatially periodic media is devel
oped. This theory takes account of distortions of the atomic wave functions
in the field of an external electromagnetic wave and also of the spatial d
ispersion. The results of experiments on second-harmonic generation in one-
dimensional periodic media are discussed. The angular and polarisation depe
ndences of the reflected second harmonic are calculated. A comparison of th
e experimental results with the theoretical calculations shows that they ar
e in good agreement.