Holter (ambulatory) monitoring initially was used to detect cardiac arrhythmia. With increasing sophistication of recording hardware, it is now possible to detect ST-segment changes indicative of myocardial ischemia. Programs are also available to detect abnormal changes in the circadian variability of cardiac rate. At present, Holter monitoring is used both in detection of arrhythmia and in monitoring efficacy of treatment. Overt and silent myocardial ischemic episodes in patients can be detected and are powerful predictors of future serious illness. The predictive value of heart rate variability is less certain. The highest return of information comes from monitoring patients at highest risk of disease”i.e., those with coronary artery disease, unexplained syncope, or known cardiac arrhythmias. It should be noted that 24-hour monitoring may not be sufficient to truly detect disease and that ambulatory recordings should always be correlated with the standard ECG.