In ambulatory ECG registration the patient's heart is continuously monitored during the daily activities with a portable ECG-recorder. It is used for the investigation of heart arrhythmias (tachy- and bradycardias). Modern digital recording devices are smaller and more reliable than the cassette recorders that were earlier used. In addition, they allow longer recording times than earlier. The most commonly used devices are a Holter monitor (1-2 days), an event recorder (3-7 days), tele-ECG (1 week to 3 months) and an implantable loop recorder (12-36 months). In event-ECG and tele-ECG the recording is patient-activated when symptoms occur. In children, ECG must always be interpreted in respect to the age of the child. Modern recorders of twelve lead ECG often give a summary of the findings. The reliability of the interpretation varies markedly depending on the reference values used. The interpretation may be misleading, especially when the rhythm is evaluated. The summary often includes a comment like 'possible hypertrophy'. This may be due to high voltage measured in the recording of a single lead. Thus, the physician must always evaluate the ECG as an entity.