Twelve-lead electrocardiograms (ECGs) record 12 different views of cardiac electrical activity, or leads. They therefore provide more information than bedside monitors, which usually record one or sometimes two leads. Many acutely ill patients have 12-lead ECGs recorded either on admission to hospital, before undergoing operations or when specific cardiac concerns have arisen. Traditionally, ECGs have aided clinical diagnoses and, by learning how to understand and interpret them, nurses will know when they need to summon expert help. 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 in interpreting them.