Differential reflectance (DR) spectroscopy is similar to other optical modulation techniques in so far as the resulting spectra exhibit sharp derivative-like lineshapes at photon energies corresponding to the critical point transitions. DR signals originate from inhomogeneities on or below the semiconductor surface. These inhomogeneities may be intrinsic, such as fluctuations in surface field, layer thickness, alloy composition, or externally induced, such as ion implantation, hydrogenation, etc. The DR spectra of semiconductor layer structures may be used to determine mole fraction, doping concentration, critical point energies, etc., much like photoreflectance (PR). In many cases, DR spectra have better signal-to-noise ratio than that of the PR spectra. In this report we discuss the application of DR in the study of doping inhomogeneities in GaAs as well as the use of DR to determine damage profiles in ion implanted GaAs.