The use of optical heterostructures for nonlinear optics and in particular frequency conversion is reviewed. A key issue for frequency conversion is the possibility of phase matching, that is to compensate for optical dispersion, which results in different phase velocities for light of different frequencies. Various one-dimensional and two-dimensional optical heterostructures, which can be used for phase matching nonlinear interactions, are described. Among these structures,
photonic crystals with
is periodic, are distinguished.