The carbonic anhydrase (CA) class of enzymes are amongst the most abundant in nature and are primarily responsible for the reversible hydration of carbon dioxide to bicarbonate. In humans there are many different isozymes of CA that vary in sub-­cellular and tissue distribution. Some isozymes such as hCA II (h=human) are ubiquitous, whereas others including hCA IX and XII play a major role in cancer cell proliferation and are found almost exclusively in hypoxic tumours. Through the design of small molecules that explore a range of structure-­activity and structure-property relationships (SAR and SPR respectively), inhibitors capable of selectively targeting a specific CA isozyme may therefore offer a series of new therapeutic targets.