Plastids are a diverse group of organelles found in plants and some parasites. Chloroplasts are the archetypical plastids and are present in photosynthetic plant cells. Because most plastid proteins are encoded by the nuclear genome, plastid biogenesis relies on importing these proteins into the plastid. On the other hand, changes in functional or metabolic states of plastids have been known to affect the expression of nuclear genes encoding plastid proteins, and are collectively called "plastid signals." This regulation is also important for maintaining plastid function. This review focuses on the roles of these anterograde and retrograde pathways in plastid biogenesis and environmental adaptation.