The carbonic anhydrase (CA, EC 4.2.1.1) isozymes IX and XII are predominantly found in tumor cells and show a restricted expression in normal tissues. By efficiently hydrating carbon dioxide to protons and bicarbonate, these CAs contribute significantly to the extracellular acidification of solid tumors. CA IX and XII are overexpressed in many such tumors in response to the hypoxia inducible factor (HIF) pathway, and research on the involvement of these isozymes in cancer has progressed in recent years. The report of the X-ray crystal structure of CA IX, which is a dimeric protein with a quaternary structure not evidenced earlier for this family of enzymes, allows for structure-based drug design campaigns of inhibitors against this novel antitumor target. Indeed, it has been known for some time that aromatic/ heterocyclic sulfonamides and sulfamates have good affinity for this isoform, but generally they do not show specificity for the inhibition of the tumor-associated isoform versus the remaining CA isozymes (CA I-VII, and XII-XV) found in mammals. Recently, we reported several classes of compounds with good selectivity for the tumor-associated CAs, being shown that CA IX/XII inhibition reverses the effect of tumor acidification, leading to inhibition of the cancer cells growth. CA IX/XII are now proposed as novel therapeutic antitumor targets. Furthermore, as some types of CA inhibitors (CAIs), such as the fluorescent sulfonamides accumulate only in hypoxic tumor cells overexpressing these enzymes, CAIs may be also used as diagnostic tools for imaging of hypoxic cancer cells. Work from several laboratories recently reported the proof-of-concept studies for the use of CA IX/XII inhibitors as well as antibodies both in the therapy and imaging of hypoxic tumors.