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Ion and acid–base regulating mechanisms have been studied at the fish gill for almost a century. Original models proposed for Na⁺ and Cl⁻ uptake, and their linkage with H⁺ and HCO₃⁻ secretion have changed substantially with the development of more sophisticated physiological techniques. At the freshwater fish gill, two dominant mechanisms for Na⁺ uptake from dilute environments have persisted in the literature. The use of an apical Na⁺/H⁺ exchanger driven by a basolateral Na⁺/K⁺-ATPase versus an apical Na⁺ channel electrogenically coupled to an apical H⁺-ATPase have been the source of debate for a number of years. Advances in molecular biology have greatly enhanced our understanding of the basic ion transport mechanisms at the fish gill. However, it is imperative to ensure that thermodynamic principles are followed in the development of new models for gill ion transport. This review will focus on …