Applications of in-situ reflectance monitoring to epitaxial growth are presented by an example of monitoring GaN metalorganic vapor phase epitaxy. Shallow-angle reflectance using ultraviolet light was used to compare the different types of growth on sapphire and on 6H-SiC substrates. By this method, stable monitoring is possible without being influenced by a strong and visible black-body radiation from the substrate heated to high temperatures. The growth-rate was successfully monitored by optical interference, and the morphological change was detected by the reflectivity change due to Rayleigh scattering. The use of p-polarized light in measuring shallow-angle reflectance that is called surface photoabsorption was applied to monitor the chemical stoichiometry of GaN surface during growth.