This review examines the mechanisms and regulation of ion and acid transfers across the gills, focussing on freshwater fish. Apical ion-exchange mechanisms (i.e., [Formula: see text] and [Formula: see text] exchange) are shown to play an important though not exclusive role in acid–base transfers. Under some circumstances, for example when either [Formula: see text] or [Formula: see text] but not H+ levels in plasma are elevated, a second mechanism of acid–base transfer then becomes apparent. This mechanism is thought to involve diffusion of acid–base relevant and "strong" ions through paracellular channels. Factors that affect acid–base transfers across the gills include the external [NaCl] and the nature of the acid–base disturbance, the latter being at least as important as the magnitude of the disturbance. Catecholamines are shown to play an important role in the regulation of acid–base transfers across the gills, acting largely through activation of β-adrenergic receptors. Current evidence, nonetheless, suggests that other elements (e.g., α-receptors, adrenergic and cholinergic neurons) may also be involved.