1. Errors arise when an action is intended but not performed; errors that arise from poor planning or inadequate knowledge are characterized as mistakes; those that arise from imperfect execution of well-formulated plans are called slips when an erroneous act is committed and lapses when a correct act is omitted. 2. Some tasks are intrinsically prone to error. Examples are tasks that are unfamiliar to the operator or performed under pressure. Tasks that require the calculation of a dosage or dilution are especially susceptible to error. 3. The tasks of prescribing, preparation, and administration of medicines are complex, and are carried out within a complex system; errors can occur at each of many steps and the error rate for the overall process is therefore high. 4. The error rate increases when health-care professionals are inexperienced, inattentive, rushed, distracted, fatigued, or depressed; orthopaedic surgeons and nurses may be more likely than other health-care professionals to make medication errors. 5. Medication error rates in hospital are higher in paediatric departments and intensive care units than elsewhere. 6. Rates of medication errors may be higher in very young or very old patients. 7. Intravenous antibiotics are the drugs most commonly involved in medication errors in hospital; antiplatelet agents, diuretics, and non-steroidal anti-inflammatory drugs are most likely to account for 'preventable admissions'. 8. Computers effectively reduce the rates of easily counted errors. It is not clear whether they can save lives lost through rare but dangerous errors in the medication process.