Modulated reflectance is a straightforward technique in which the reflectivity spectrum of a semiconductor is perturbed by an external periodic influence-most usefully a chopped laser beam, i.e. photo-modulated reflectance (PR). Since samples need no special mounting, can be studied in air at room-temperature and can be full-sized pre-fabrication wafers, PR is truly non-destructive. In contrast to conventional photo-luminescence, PR spectra are often replete with sharp, detailed derivative-like features arising from ground-state, and other possible higher-energy optical transitions within a sample, from which can be extracted material parameters crucial to successful and efficient device operation, such as compositions, layer thicknesses, built-in electric fields and band line-ups. The paper discusses the advances made by our group in the application and interpretation of modulation spectroscopy of vertical-cavity surface-emitting lasers and resonant-cavity light-emitting diodes, which so far have defied analysis by the more conventional non-destructive techniques. (C) 2003 Elsevier B.V. All rights reserved.