The number of people over 65 is increasing and those over 65 requiring surgery will likewise increase. Post-operative delirium (POD) affects up to 47% of patients undergoing surgery and is more prevalent in older people. Importantly, POD is associated with increased morbidity, mortality, length of stay and care home placement. There is evidence that specialist geriatrician input reduces POD but to be cost effective, needs to target patients with increased risk for POD. Many factors have been associated with increased risk of POD, including age, cognitive impairment, gender, depression, alcohol, drug use, smoking, co-morbidity, functional status, ASA score and pre-operative biochemical and haematological abnormalities. This article reviews the literature associated with the above factors, considers frailty as a factor and also suggests that POD may be associated with rapidity of onset and severity of the insult to the patient.