Chloroplasts have developed a light-dependent system for the control of the activities of key enzymes involved in assimilatory (photosynthetic) and dissimilatory pathways, which allows a switch between these opposing pathways to prevent futile cycling. This regulatory system, known as the ferredoxin/thioredoxin system, consists of several proteins constituting a redox cascade that transmits the light signal perceived by chlorophyll to selected target proteins, thereby influencing their activity. A central component of the redox cascade is a novel enzyme, the ferredoxin:thioredoxin reductase, which is capable of reducing a disulfide bridge with the help of an iron-sulfur cluster. Recent developments on the elucidation of the structures of several implicated proteins and on the mechanism of signal transfer have greatly improved our understanding of this regulatory mechanism. This review describes the components of the redox cascade, the principal target proteins, and the mechanism of action of the light-signal transfer.