Postoperative cognitive dysfunction (POCD) is defined as a cognitive decline occurring after surgical intervention. POCD must be distinguished from postoperative delirium, the latter syndrome being characterized by a disrupted sleep-wake rhythm, variable levels of consciousness and altered psychomotor activities. The exact diagnosis of POCD is difficult to make, since the presurgical cognitive state needs to be assessed to confirm a significant change of mental status after surgery. The incidence varies between 0% and 66%, depending on the test procedures used and the patient group. Like postoperative delirium syndrome, POCD is associated with an increased risk of mortality. An early phase within the first week is distinguished from a late phase. Increasing age and co-morbidity have been identified as important risk factors, as have extensive surgical interventions and long-duration anaesthesia. To prevent POCD, surgical trauma should be minimized (e.g. with minimally invasive techniques), while short-acting anaesthetics may be of great benefit. Pro-inflammatory cytokines or metabolites in the central nervous system may play a major role in the development of POCD.