This review deals with the biophysical aspects of flavin-based photosensors, comprising cryptochromes, LOV (Light, Oxygen and Voltage) and BLUF (Blue Light sensing Using FAD) proteins. Special emphasis is given to structural issues, photocycle quantum yields and energetics, mechanism of the lighttriggered reactions, early stages in signal transduction and oligomeric states of the light sensing protein modules. For BLUF and LOV domains important parallels are emerging, despite their different alpha ⁄ beta fold arrangement, whereas there is increasing evidence for a mechanicistic and functional splitting of the cryptochrome family.