 This chapter focused on how cyanobacteria adapt to varying levels of carbon dioxide, CO2. It examined the mechanisms of inorganic carbon, CI, uptake, photorespiration, and the regulation of metabolism under different conditions. Additionally, it discussed the structure of various transport and uptake systems, as well as the genetic regulations behind them. Furthermore, it explored the role of small molecules in regulating gene expression and enzyme activity, and discussed the potential involvement of small non-coding RNA, SNCRNA, in further regulation. This article was authored by Robert L. Burnup, Martin Hageman, and Aaron Kaplan.