Plastid proteins are encoded separately on two genomes, one in the nucleus and another in the plastid. Thus, expression of genes on both genomes should be coordinated in response to developmental and environmental cues. Many nuclear-encoded factors are involved in transcription regulation of plastid genes. On the other hand, some plastid-derived signals also regulate expression of specific sets of nuclear genes. In this chapter, we summarize and discuss our current views of regulation of plastid gene expression, especially emphasized on the nucleus–plastid communication. The regulatory systems in red algae and green plants are likely to be rather different from each other.