About 50 years ago, Norman Jefferis Holter invented a device that opened the possibility of recording heart activity over long periods of time. This invention, together with the rapid developments in electronics, has enabled a revolutionary change in the diagnosis and management of cardiac diseases. Ambulatory cardiac monitors have decreased in size to the point of becoming wearable or implantable and are able to monitor heart activity for months or even years. In addition, new telecommunication systems allow clinicians to remotely access cardiac events and to respond within a short period of time. Novel advances in computing and algorithm development are expanding the clinical applications of ambulatory devices with more complex automatic interpretation of the electrocardiographic signal. This article reviews the state of the art of these techniques from both clinical and technical approaches, covering a historic perspective up to today, and discusses current applications, challenges, and future directions.