Potentially inappropriate prescribing (IP) is an issue of major importance in the pharmacotherapy of older people globally. To date, Beers’ criteria have been in common research usage for defining potentially inappropriate medications (PIMs) in older people. However, Beers’ criteria have a number of serious flaws and are of doubtful relevance to routine geriatric pharmacotherapy. For these reasons, new geriatric IP criteria have been devised and validated, called screening tool of older persons’ prescriptions (STOPP) and screening tool to alert to right treatment (START) for detection of potential errors of prescribing commission and omission. Prevalence studies in older people in southern Ireland show substantial rates of prescription of one or more PIMs in the primary care setting (22%), in acute hospital care (35%) and in nursing home care (60%) using STOPP criteria. Similarly, prevalence rates of potentially serious prescribing omission are high both in hospital (58%) and in primary care (23%). Prospective data show that STOPP criteria detect adverse drug effects (ADEs) that are causal or contributory to acute hospitalisation in older people 2.8 times more frequently than Beers’ criteria. This suggests that ADEs are likely associated with PIMs in older people, contrary to recent research data showing no significant association on the basis of defining PIMs using Beers’ criteria. We have recently shown that STOPP/START criteria as an intervention significantly improves medication appropriateness in hospitalised older people. Whether STOPP/START can prevent ADEs and reduce drug costs remains to be elucidated by means of further randomised controlled trials.