Ambulatory 24 hour monitoring was developed in the United States and bears the name of its inventor, Holter. It was introduced in France for the first time by the Lariboisière school, 25 years ago, and the end of the year 2000 is an appropriate time to review this tool. What are the new clinical indications, technical progress and prospects for the future? All the efforts of both cardiologists and engineers for the past 25 years have been focused on gathering information about the system of cardiac regulation which is more or less concealed in the ECG recordings. As far as arrhythmias are concerned, or the QRS or T wave morphology, or the circadian variations of sinus rate and depolarisation, the studies are numerous but not always easily applicable in clinical practice. However, the non-invasive analysis given by the Holter recording guarantees a promising future in terms of new discoveries and applications.