Photosynthesis within chloroplasts is crucial for ecosystem function and all agricultural productiv- ity. Chloroplasts are a type of plastid and contain a small circular genome of prokaryotic origin. This genome encodes proteins crucial for chloroplast function and requires correct regulation of gene expression. One cellular mechanism regulating plastid gene transcription involves sigma factors, which in plants are nuclear-encoded proteins form- ing part of the chloroplast transcriptional system. These are required for chloroplast gene promoter recognition and transcription initiation, with spe- cific sigma factors thought to recognise specific chloroplast promoters. Plant sigma factors partici- pate in the adjustment of chloroplast transcription in response to environmental fluctuations and dur- ing development. They appear to be ancient, orig- inating from photosynthetic bacteria that were chloroplast ancestors, with an increase in sigma factor copy number during plant evolution provid- ing an example of the evolution of cell signalling. We examine the origins, structure, function and environmental signalling by plant sigma factors.