Medication errors cause substantial harm to patients. We need good methods for counting errors, and we need to know how errors defined in different ways and ascertained by different methods are related to the harm that patients suffer. As errors arise within the complex and poorly designed systems of hospital and primary care, analysis of the factors that lead to error, for example by failure mode and effects analysis, may encourage better designs and reduce harms. There is almost no information on the best ways to train prescribers to be safe or to design effective computerized decision support to help them, although both are important in reducing medication errors and should be investigated. We also need to know how best to provide patients with the data they need to be part of initiatives for safer prescribing.