Recent developments in glow discharge optical emission spectrometry (GD-OES) resulted in establishing this technique as a powerful tool for quantitative elemental depth profile analysis of coating-substrate systems and surface-modified materials. The present status of GD-OES as an analytical discipline is reviewed, methodology of quantitative depth profile analysis is described and the resulting analytical performance is presented on examples of PVD/CVD hard coatings on cemented carbides and high-speed steels. It is shown that with GD-OES, it is possible to analyze the depth-resolved elemental composition of the coating and the substrate in a relatively simple experimental setup and with an accuracy approaching the bulk analysis of homogeneous materials.