Photosynthetic prokaryotes have highly developed abilities to detect and react to environmental signals. Light sensing is one of the most important capabilities of organisms that use light for photosynthesis and photomorphogenesis. This review addresses photoreception in cyanobacteria from the perception of light through the physiological responses observed in response to light-dependent signalling. Recent progress made in our understanding of the structure and function of photosensory receptors and their downstream effector molecules is discussed.