Continuous ECG by the Holter method provides valuable information on the electrical activity of the heart but the interpretation of the playback may be made difficult by the presence of artefacts. The principal artefacts which may mimic arrhythmias or blocks, disturb QRS detection or deform the ventricular depolarisation are described. The majority are caused by incomplete preparation of the patient or the electrodes, but others may arise from a mechanical or electrical fault. Artefacts interfere with the automatic analysis of the tape, which is one reason for knowing the limitations of these systems of automatic analysis, and therefore control visual reinterpretation is necessary in cases where there is the slightest doubt. The presence of these artefacts is a limitation of the Holter method, the practice of which demands a certain amount of experience to avoid incorrect diagnoses which could lead to inappropriate or injustified therapy.