The membrane-bound nicotinamide nucleotide transhydrogenase is a key enzyme for the maintenance of metabolic balance in mammalian cells as well as in many bacteria. The enzyme resides in the mitochondrial inner membrane in eukaryotic cells or the cytoplasmic membrane in bacteria. Under normal physiological conditions, the transhydrogenase utilizes the proton motive force to drive hydride transfer from NADH to NADP⁺, thus generating NADPH. Among other functions, NADPH is critical for the cellular defense against reactive oxygen species. Although not the only source of NADPH, the transhydrogenase is often important, depending on cell type and physiological state. People with the most severe mutations in the Nnt gene, encoding transhydrogenase, suffer from familial glucocorticoid deficiency. Recent X-ray structures of the transhydrogenase from the hyperthermophilic bacterium Thermus thermophilus have provided key insights into how this enzyme couples proton flux across the membrane to hydride transfer. The central hypothesis from these studies focuses on the proposal that large motions of the NADP(H) binding domain (dIII), swiveling between alternating states during the catalytic cycle, are responsible for gating the proton channel in response to the redox state of bound NADP⁺/NADPH.