In this compilation, the authors address the relationship between carbonic anhydrase IX and hypoxia. Hypoxia is a key factor influencing a life-cycle of CA IX, including its expression, correct structure, stability, function, and subsequent biological implications. Hypoxia also regulates CA IX expression by post-transcriptional mechanisms through splicing and mRNA stabilization. A review is provided focusing on the sequence of elementary reactions ensuing a catalytic reaction (catalytic mechanism) of carbonic anhydrase. The closing chapter discusses carbonic anhydrases in chloroplasts, especially their location in the organelles, as well as considers their uses.