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Potassium (K+) is the most abundant ion in the plant cell and is required for a wide array of functions, ranging from the maintenance of electrical potential gradients across cell membranes, to the generation of turgor, to the activation of numerous enzymes. The majority of these functions depend more or less directly upon the activities and regulation of membrane-bound K+ transport proteins, operating over a wide range of K+ concentrations. Here, we review the physiological aspects of potassium transport systems in the plasma membrane, re-examining fundamental problems in the field such as the distinctions between high- and low-affinity transport systems, the interactions between K+ and other ions such as NH4+ and Na+, the regulation of cellular K+ pools, the generation of electrical potentials and the problems involved in measurement of unidirectional K+ fluxes. We place these discussions in the context of recent discoveries in the molecular biology of K+ acquisition and produce an overview of gene families encoding K+ transporters.