This article describes a systematic review on the research into postoperative cognitive dysfunction (POCD) in noncardiac surgery to ascertain the status of the evidence and to examine the methodologies used in studies. The review demonstrated that in the early weeks after major noncardiac surgery, a significant proportion of people show POCD, with the elderly being more at risk. Minimal evidence was found that patients continue to show POCD up to 6 months and beyond. Studies on regional versus general anesthesia have not found differences in POCD. Many studies were found to be underpowered, and a number of other methodologic difficulties were identified. These include the different types of surgery in studies and variations in the number and range of neuropsychological tests used. A particular issue is the variety of definitions used to classify individuals as having POCD.