The teleostean gill is a multi-purpose organ, specialized for respiratory gas exchanges, clearance of waste products of nitrogenous metabolism and maintenance of acid-base and mineral balances. Structural studies reveal a complex epithelium. The 'chloride-cells are almost certainly the site of ion exchange in relation to salt balance. Functional studies show that the gill is responsible for the net absorption of Na<latex>$^+$</latex> and Cl<latex>$^-$</latex> occurring in fresh water and extrusion of these ions in sea water. In fresh water, a coupling between endogenous NH<latex>$^+_4$</latex> or H<latex>$^+$</latex> and HCO<latex>$^-_3$</latex> excretion and Na<latex>$^+$</latex> and Cl<latex>$^-$</latex> absorption is observed. In sea water active Na<latex>$^+$</latex> excretion is linked with K<latex>$^+$</latex> absorption from the external medium. In parallel, active Cl<latex>$^-$</latex> excretion occurs. The gill is also the site of Na<latex>$^+$</latex>/Na<latex>$^+$</latex> and Cl<latex>$^-$</latex>/Cl<latex>$^-$</latex> exchanges which involve 25 to 75% of the internal NaCl per hour. The relative importance of simple diffusion and exchange-diffusion in these exchanges is assessed. Biochemical studies reveal two enzymes playing important roles in the ionic pumps: carbonic anhydrase and Na-K activated ATPase. Studies involving transfer of euryhaline fishes from low to high salinity, show that the switch from fresh-water to seawater types of gill function is far from instantaneous. Synthesis or destruction of functional sites and renewal of specialized cells are involved. The role of external or internal NaCl concentration changes as stimuli for these 'inductive processes' and the endocrine control of these functional changes are briefly discussed.