Diatoms are a group of unicellular eukaryotic microalgae of high evolutionary success and of ecological importance. Like all photosynthetic organisms, they need to tightly regulate their photosynthetic machinery to both generate enough energy in low light conditions and avoid photodamage by absorption of excess light energy. Many other aspects of diatom biology, from central metabolism and cell cycle to migration patterns, have been shown to be influenced by light of certain wavelengths. One possible mechanism to determine both light quantity and light wavelength is the useof photoreceptors. The genomes of diatoms encode up to four different classes of photoreceptors: (1) Phytochromes are a class of photoswitchable red/far red-light absorbing photoreceptors, which are evolutionary widespread; (2) Blue-light sensing cryptochromes, which share a high homology to DNA-repairing photolyases, are also evolutionary widespread; (3) Aureochromes, a group of blue-light dependent transcription factors which were identified in 2007 and are restricted to the Stramenopiles. (4) Heliorhodopsins, a novel class of green-yellow light-absorbing photoreceptors identified in 2018 from a metagenome study, have had their domain topology reversed compared to previously known Rhodopsins. In this chapter, we summarize the mode of action of each of these photoreceptors with examples from what is known from other species, such as plants, give an update on their phylogenetic distribution among diatom species and describe what is known about the respective photoreceptors in diatoms both using expression data from microarray and RNA-seq experiments and functional data derived from overexpression, knockdown and knockout strains.