Carbonic Anhydrase (CA) XII, also known as Car12 and CA12, is an extracellular enzyme involved in the regulation of the microenvironment acidity and tumor malignant phenotype, was originally identified as a protein overexpressed in some types of cancers. It has showed that CA XII is induced by hypoxia and oestrogen and expressed at high levels on various types of cancer. The enzyme is directly involved in tumour progression, and its inhibition has an anti-tumour effect. Apart from its role in carcinogenesis, the enzyme contributes to various other diseases like glaucoma and arteriosclerotic plaques, among others. CA XII is therefore regarded as promising target for specific therapies, and may be used as a novel prognostic marker in combination with histologic grade of the tumors.