Light provides the energy used in photosynthesis and the signals used in photoregulation of plant growth and development, and among the factors affecting plants is perhaps the most spatially and temporally heterogeneous. Leaves are the most specialised above ground organ for resource capture in plants. They harvest light, but also CO₂ for photosynthesis, and the origins of leaves seems to be linked to the enhanced harvest of the latter and not of the former. However, there is consensus on the fact that the evolution of leaf anatomy, morphology and function has been shaped by light capture, at least in a significant part. Even though certain topics such as the morphology and physiology of sun vs. shade leaves have been intensively studied, a number of important aspects remains to be explored and connections need to be made among the different disciplines involved (mainly biochemistry, physiology, genetics, morphology, ecology, and evolutionary biology). For instance, the fact that terrestrial animals are very sensitive to the greatest radiant energy (green region of the spectrum), while terrestrial plants have a pigment system that absorbs the least in the same spectral region (i.e. why are leaves green?) is evolutionarily and ecologically intriguing. Despite this incomplete picture, a lot of information can be extracted from the colour of leaves: the characteristic spectral signature of the light reflected on leaves, coupled with an adequate ecological and physiological information, can be used to quantify by remote sensing relevant  ecosystem properties such as radiation-use efficiency, productivity, water content (and thus, risk of fire), and level of stress of the vegetation. Global radiation is decreasing, which coupled with other aspects of global change is producing new abiotic scenarios. The understanding of light as an ecological factor can be crucial to predict the evolution of natural systems: e.g. competitive advantages conferred by leaf-level plastic responses to light seems to be involved in the invasive nature of certain plants, and the complex interrelationships between light and water availabilities can lead to counterintuitive changes in productivity and species interactions.