The optical properties of the group-III-nitride materials are obviously of direct relevance for optoelectronic applications, but experiments measuring optical properties also give information on a range of electronic properties. The optical properties of bulk GaN have been studied by several authors by spectrometric ellipsometry, yielding the entire dielectric function over a wide range from deep UV to infrared. GaN is a direct-bandgap semiconductor with the conduction band minimum and valence band maximum both occurring at the center of the Brillouin zone. The basic optical properties of GaN in the near-bandgap region are known with some precision since more than a decade. Absorption data from transmission experiments are less straightforward to measure, due to the strong oscillator strengths of the excitons, requiring a sample thickness <1 µm. The presence and the quantum efficiency of a particular emission depend on the method used for growth of GaN material, the growth conditions, as well as on the concentration of the impurities.