Cardiac electrical abnormalities are common, and result in considerable morbidity and mortality. Although diagnosis can sometimes be made from the resting 12-lead ECG, a longer period of monitoring is often required to capture diagnostic information. Ambulatory ECG (AECG) devices allow cardiac rhythm to be monitored and recorded over days, weeks or years, and are used primarily in the outpatient setting. Multiple devices are available, with a variety of features. Recording may take place continuously or occur intermittently in response to patient activation or auto-sensing of rhythm disturbances. Devices may be fitted externally or implanted below the skin. Data may also be acquired from an existing pacemaker or implantable cardioverter defibrillator (ICD). Commonly used methods of AECG monitoring are described and appraised in this article and newer developments in monitoring are evaluated, including the role of patient-led, smartphone-based devices.