The carbonic anhydrases play important roles in respiration, acid-base homeostasis, bone resorption, calcification, photosynthesis, several biosynthetic pathways and a variety of processes involving ion, gas and fluid transfer. This text examines the role of carbonic anhydrases. The enzyme is present in three known gene families: the a-CAs (animals and algae) of which seven isozymes have been characterized; the b-CAs (plants and bacteria); and the recently discovered g-CAs (archaebacteria). In addition to their clinical and applied physiological importance, the carbonic anhydrases have become accepted as a model for the study of evolution of gene families and also used as model systems in site-directed mutagenesis in structure/function relationships, protein folding and transgenic and gene target analysis.