Potassium is an essential plant nutrient. This chapter describes the physiological roles of potassium: in activating enzymes, as a cellular osmoticum, and as a counter cation for anion accumulation and electrogenic transport processes. The chapter then describes the molecular biology of potassium uptake and redistribution in plants. The chapter highlights the contrasting potassium requirements of different crops and describes substantial heritable variation within crops in the ability to acquire potassium from the soil and utilise it physiologically to produce yield. The chapter describes the symptoms of potassium deficiency in plants and the acclimatory responses of plants to low potassium supply. The chapter concludes with a summary of the traits that might be selected in breeding crops that require less potassium fertiliser, and looks ahead to future research trends in this area.