Book excerpt: Experimental work emphasized demonstrating the use of high resolution frequency domain nonlinear laser spectroscopy for the study of materials, particularly semiconductor heterostructures. The work has demonstrated that cw four-wave mixing spectroscopy provides new information regarding the origin of the nonlinear optical response in semiconductor materials as well as new information about structure and relaxation. A summary of the current progress shows: (1) First experimental observation of an interference effect in the nonlinear optical response in GaAs quantum well structures corresponding to a slow component in the nonlinear optical response. The interference effect is evidence that the exciton resonance frequency is shifted in the presence of the electron hole plasma produced by the ionized exciton. (2) Demonstration of frequency domain nonlinear optical spectroscopy methods for the direct measurement of the ambipolar diffusion coefficient and the electron-hole recombination rate in GaAs quantum well structures. (3) Measurement of the response time in strained quantum well structures where the heavy and light hole exciton overlap in energy. and (4) Use of precision nonlinear optical spectroscopy methods for the study of phonon induced spectral diffusion of the heavy hole exciton at low temperature in GaAs quantum well structures. Keywords: Semiconductor junctions; Nonlinear optical analysis. (cp).