Retrograde signaling coordinates the expression of nuclear genes encoding organellar proteins with the metabolic and developmental state of the organelle. These plastid signals are essential not only for coordinating photosynthetic gene expression in both the nucleus and in the chloroplasts but also for mediating plant stress responses. The chloroplasts therefore act as sensors of environmental changes and complex networks of plastid signals coordinate cellular activities and assist the cell during plant stress responses. Recent work suggests that information from both cytosolic-signaling and plastid-signaling networks must be integrated for the plant cell to respond optimally to environmental stress.