Postoperative cognitive dysfunction (POCD) is a subtle impairment of memory, concentration, and speed of information processing. It is a frequent complication following surgery and can have a debilitating effect on patients' recovery and future prognosis. Neuropsychological testing is needed to reveal postoperative cognitive decline, and questionnaires are not useful for this purpose. There is a profound lack of consensus regarding the research methodology for detection of cognitive deterioration, especially the diagnostic criteria. Issues, such as baseline performance, learning effects, and the interval between test sessions, also influence test results and must be considered when designing and interpreting POCD data.