The development and widespread application of 24-hour ambulatory ECG monitoring in the last 15 years represent major advances in cardiology. It is the most sensitive method of detecting arrhythmias and allows objective evaluation of such abnormalities and their response to treatment as the patient continues the activities of daily life. Information on prognosis and early diagnosis in patients with coronary artery disease has been obtained but more data and a greater understanding of their relevance in normal populations are necessary. Until then the significance of the findings in many situations, especially in asymptomatic patients, will remain speculative and decisions on whether or not to institute therapy will continue to be somewhat arbitrary.