The efficiency of photosynthesis is a crucial parameter for the functional state of chloroplasts. Many environmental factors affect photosynthetic performance. Therefore, the states of partial reactions of photosynthesis are sensed and employed to organize adaptive responses to environmental changes. Components of the photosynthetic electron transport chain undergo significant changes in their reduction/oxidation (redox) state depending on the balance between light absorption and acceptor availability and serve as signaling parameters, which report environmental influences on photosynthesis. Redox signaling pathways are triggered by the redox state of the plastoquinone pool, the acceptor availability at photosystem I, the thioredoxin system, and the thiol-/disulfide redox state of target proteins. Such redox signals are suggested to play an important role in the co-ordination of chloroplast and nuclear gene expression events. It is a topic of intensive current research efforts to characterize the photosynthetic parameters that control nuclear gene expression and to unravel the signal transduction mechanisms transmitting information to the nucleus.