Optogenetics is an emerging field that combines optical and genetic approaches to non-invasively interfere with cellular events with exquisite spatiotemporal control.Although it arose originally from neuroscience,optogenetics is widely applicable to the study of many different biological systems and the range of applications arising from this technology continues to increase.Moreover,the repertoire of light-sensitive proteins used for devising new optogenetic tools is rapidly expanding.Light,Oxygen,or Voltage sensing (LOV) and Blue-Light-Utilizing flavin adenine dinucleotide (FAD) (BLUF) domains represent new contributors to the optogenetic toolkit.These small (100-140-amino acids) flavoprotein modules are derived from plant and bacterial photoreceptors that respond to UV-A/blue light.In recent years,considerable progress has been made in uncovering the photoactivation mechanisms of both LOV and BLUF domains.This knowledge has been applied in the design of synthetic photoswitches and fluorescent reporters with applications in cell biology and biotechnology.In this review,we summarize the photochemical properties of LOV and BLUF photosensors and highlight some of the recent advances in how these flavoproteins are being employed to artificially regulate and image a variety of biological processes.