Publisher Summary The biosynthesis of carotenoids is affected by light in bacteria, fungi, algae, and higher plants. However, the extent to which synthesis is regulated by light differs from organism to organism. In some fungi like Phycomyces, certain amounts of carotenoids are produced in the dark, and light increases the level of pigments. To identify the molecule that serves as a photoreceptor for carotenogenesis in a particular organism, action spectra have to be determined. The procedure for obtaining such action spectra is described in this chapter. The procedures for illumination of samples and direct measurement of light-induced pigment formation via spectroscopic analysis are discussed in the chapter. In Verticillium agaricinum, parallel to the light-dependent carotenoid synthesis, light induces protein synthesis, as shown as an increased activity of 80S ribosomes. Finally, direct activation of a step in the carotenogenic pathway by phototransformation of a compound that leads to the onset of activity of preexisting enzymes is demonstrated in the chapter in algal mutants of Chlorella and Scenedesmus.