The advantages of spectroscopic ellipsometry and spectral reflectivity are combined in differential reflectometry, in which the normalized difference between the reflectivities of two specimens is measured as a function of photon energy. The main advantage of DR lies in its ability to eliminate any undesirable influences of oxides, windows, electrolytes, instrumental variations, etc. Owing to the differential nature of the technique, no vacuum is needed. Thus, surface layers can be studied in situ. Some applications are presented in this paper.