The physics of buried solid/solid interfaces is interesting for both fundamental and technological reasons. In this thesis, second-order nonlinear optical spectroscopy was used to probe the electronic structure of GaAs based interfaces in the near infrared. Second-order nonlinear optical spectroscopy possesses intrinsic interface sensitivity and a long penetration depth. This intrinsic interface sensitivity enables us to suppress spectral contributions from the adjoining bulk media, resulting in more specific assignments of the spectral features. Our observations provide strong evidence for the existence of atomic displacement induced defect states at the buried metal:GaAs interface. In particular, in Au:GaAs n-type systems we have observed two resonance features at 0.715 eV and 0.731 eV for the Ga-rich interface and a single resonance feature at 0.715 eV for the As-rich interface. Similar single resonance features at 0.715 eV were observed in As:GaAs n-type samples, but were not present in oxide:GaAs and metal:GaAs p-type systems. After consideration of various three-step optical processes, we conclude that the transitions originate from the midgap states. These resonances differ substantially from what has been observed by other less interface sensitive technologies probing similar system. The origin of these difference is discussed. In the case of the native oxide GaAs system, we also studied depletion electric field induced effects on second-order nonlinear optical processes. Using a simple three-band model, we calculated the depletion field induced perturbation of the second-order nonlinear susceptibility of GaAs(OOl). The field induced modification of the second-order nonlinear susceptibility has been found to add destructively to the unperturbed existing bulk nonlinear second-order susceptibility χ(2)/xyz. This diminution depends on the square of the near surface depletion electric field. Through studies as a function of dopant type and concentration, we demonstrate that the bulk χ(2)/xyz is systematically reduced as a result of the depletion field, and that this diminution depends on the square of the near surface depletion field. This is consistent with our theoretical calculations. The sensitivity of the near surface bulk second-order susceptibility to the depletion electric field has also been demonstrated by a photomodulation second harmonic generation (PSHG) technique, by which we measured surface minority trap lifetimes and transverse diffusion of carriers confined near GaAs(OOl) surfaces.