Three-wave mixing (3WM) spectroscopy is an exciting and relatively unexplored probe of buried solid interfaces. It possesses long penetration depths characteristic of most optical methods and intrinsic interface specificity characteristic of second-order optical processes. In this thesis we present frequency domain measurements of the ZnSe/GaAs(OOl) heterojunction by second-harmonic (SH) and sum-frequency (SF) generation. Our experiments reveal an unusual three-wave mixing resonance that arises as a result of virtual transitions between an interfacial quantum well state and the ZnSe valence band. The interfacial quantum well was brought about by interdiffusion of Zn (Ga) into GaAs (ZnSe) during sample growth. The observation introduces a new class of nonlinear optical phenomena at interfaces that can provide useful information about band profiles, diffusion and defects along the boundary of two semiconductors. We have found that this interfacial SH resonance is sensitive to a variety of structural phenomena. In essence any process that modifies the band profile near the junction will affect the strength of the resonance. We have observed the variation of interface SH spectra with respect to lattice strain relaxation and to surface reconstruction of the buried GaAs. In addition, using a newly developed photomodulationSHG (PSHG) technique, we have exploited this sensitivity to determine the nature and relative density of interface charge traps as a function of substrate surface reconstruction. The PSHG method was also used to study free charge trapping mechanisms at ZnSe/GaAs(OOl) heterointerfaces. Our measurements determined that the interfacial trap-centers are mainly hole-traps with lifetime of 35 sec. In the course of carrying out these experiments we also observed interference in reflected second harmonic generation from two adjoined nonlinear slabs. A theory for the phenomena was presented and was used to understand our experimental results with ZnSe/GaAs(OOl) heterostructures. This interference phenomena was introduced as a new methodology to measure the second-order susceptibility of thin overlayer materials.