Phosphate (PO) has a number of important functions in the human body and abnormalities of phosphate handling and phosphate levels are common in older people. We review the physiological functions of phosphate, the effects of phosphate deficiency and excess, and discuss the evidence underpinning management of these conditions in older people. Both hyper- and hypophosphataemia have been associated with a variety of adverse effects in different patient groups. There is currently little evidence to guide practice in treatment of abnormal phosphate levels specifically in older people. Studies looking into the effect of replacing phosphate on morbidity and mortality in milder cases of hypophosphataemia and into the dangers of, and treatment of, hyperphosphataemia in older people are required.