Light is both the energy source and the dominant signalling input for photosynthetic organisms. Accordingly, up to 30–50% of the genes expressed in plant leaves are directly or indirectly regulated by light, and virtually all aspects of plant metabolism support photosynthesis, depend upon the products of photosynthesis or are affected by photosynthesis-related changes in metabolite levels. Though not directly tied to photosynthesis per se, mitochondria support several light-associated processes, including photorespiration, nitrogen metabolism, reductant transport and the maintenance of redox balance. Recently, it has become increasingly apparent that diverse light signalling pathways intersect with the mitochondrion, and that cellular respiration and photosynthesis display tight functional and regulatory coordination. In addition, global transcript analyses performed on plants exposed to different light qualities and intensities have started to provide promising starting points for the next series of investigations of light/diurnal regulation of respiratory metabolism.