A beam of monochromatic light impinges on a surface. The lack of symmetry at the surface (or a buried interface) can lead to the generation of light at a frequency twice that of the incident light (ie the second harmonic). Surface phenomena such as moleculer adsorption, aggregation and orientation van be studied. The detected second harmonic light is of particular interest in studying buried interfaces as most surface science techniques cannot access such structures. SHG can provide information on the electric field at an interface. Related techniques are Sum Frequency Generation (SFG) and Difference Frequency Generation (DFG). SHG can also be used in association with MOKE.