Wearable cardiac monitoring devices can provide uninterrupted monitoring of cardiac activities over a long period of time. They have developed rapidly in recent years in terms of convenience, comfort, and intelligence. Aided by the development of sensor and materials technology, big data and artificial intelligence, wearable cardiac monitoring can become a crucial basis for novel medical models in the future. Herein, the basic concepts and representative devices of wearable cardiac monitoring are first introduced. Subsequently, its core technologies and the latest representative research progress in physiology signal sensing, signal quality enhancement, and signal reliability are systematically reviewed. Finally, an insight and outlook on the future development trends and challenges of wearable cardiac monitoring are discussed.