Abstract <p>Growth irradiance is a regulator of photosynthetic activity beyond the energy source. When light conditions are not ideal for photosynthesis, light becomes a stress factor that reduces the efficiency of the photosynthetic apparatus. Like other oxygenic photosynthetic organisms, cyanobacteria have developed various adaptations in response to changes in light quantity and quality. Short-term light adaptations are the first to evolve when light conditions deviate from the optimum. These serve as the first line of defense by normalizing the functioning of the pigment apparatus. They include State 1–State 2 transitions and non-photochemical phycobilisome quenching, the mechanism of which is discussed here. Additionally, short-term cyanobacterial light adaptations are compared with those that require more time and the involvement of photoreceptors.</p>