Twelve-lead electrocardiograms (ECGs) record 12 different views of cardiac electrical activity, thus providing more information than bedside monitors, which usually display one or sometimes two leads. Many acutely ill patients have 12-lead ECGs recorded on admission to hospital and at other times, such as pre-operatively or when there are specific cardiac concerns. ECGs have traditionally been an aid to medical diagnosis, but understanding and being able to interpret them enables nurses to summon help urgently if needed. This article describes what each of the 12 leads represents, outlines the main indications for recording 12-lead ECGs and identifies likely causes of error.