State 1-State 2 transitions ('state transitions') are a rapid physiological adaptation mechanism that adjusts the way absorbed light energy is distributed between photosystem I and photosystem II. They occur in both green plants and cyanobacteria, although the light-harvesting complexes involved are very different. Which aspects of the mechanism are conserved in green plants and cyanobacteria and which may be different, are discussed. It is shown that phycobilisome mobility is necessary for state transitions in cyanobacteria. A conserved cyanobacterial gene (rpaC) that plays a very specific role in state transitions has been identified. There is still debate about the physiological role of state transitions. Comparison of the growth properties of the rpaC deletion mutant with the wild-type gives us a way of directly addressing the question. It was found that state transitions are physiologically important only at very low light intensities: they play no role in protection from photoinhibition. Thus state transitions are a way to maximize the efficiency of light-harvesting at low light intensities.