Cognition may decline after surgery. Postoperative delirium, especially when hyperactive, may be easily recognised, whereas cognitive dysfunction is subtle and can only be detected using neuropsychological tests. The causes for these two conditions are largely unknown, although they share risk factors, the predominant one being age. Ignorance of the causes for postoperative cognitive dysfunction contributes to the difficulty of conducting interventional studies. Postoperative cognitive disorders are associated with increased mortality and permanent disability. Peri-operative interventions can reduce the rate of delirium in the elderly, but in spite of promising findings in animal experiments, no intervention reduces postoperative cognitive dysfunction in humans.