Cryptogenic stroke refers to ischemic stroke that is not clear to the cause of the disease through routine examinations. With the development of medical technology, studies have found that most cryptogenic stroke is caused by embolism.Therefore,the concept of embolic stroke of undetermined source (ESUS) is proposed. The main causes of ESUS include subclinical atrial fibrillation, patent foramen ovale, aortic arch atherosclerotic plaque, non-stenotic complex carotid atherosclerotic plaques, and tumor-related embolism, etc. This article reviews the etiology and secondary prevention of ESUS.