Objective: Older people experience more concurrent illnesses, are prescribed more medications and suffer more adverse drug events than younger people. Many drugs predispose older people to adverse events such as falls and cognitive impairment, thus increasing morbidity and health resource utilization. At the same time, older people are often denied potentially beneficial, clinically indicated medications without a valid reason. We aimed to validate a new screening tool of older persons' prescriptions incorporating criteria for potentially inappropriate drugs called STOPP (Screening Tool of Older Persons' Prescriptions) and criteria for potentially appropriate, indicated drugs called START (Screening Tool to Alert doctors to Right, i.e. appropriate, indicated Treatment). Methods: A Delphi consensus technique was used to establish the content validity of STOPP/START. An 18-member expert panel from academic centers in Ireland and the United Kingdom completed two rounds of the Delphi process by mail survey. Inter-rater reliability was assessed by determining the K-statistic for measure of agreement on 100 data-sets. Results: STOPP is comprised of 65 clinically significant criteria for potentially inappropriate prescribing in older people. Each criterion is accompanied by a concise explanation as to why the prescribing practice is potentially inappropriate. START consists of 22 evidence-based prescribing indicators for commonly encountered diseases in older people. Inter-rater reliability is favorable with a K-coefficient of 0.75 for STOPP and 0.68 for START. Conclusion: STOPP/START is a valid, reliable and comprehensive screening tool that enables the prescribing physician to appraise an older patient's prescription drugs in the context of his/her concurrent diagnoses.