- PURPOSE OF THE REVIEW: The purpose of the study was to review the literature on cryptogenic stroke and embolic stroke of undetermined stroke (ESUS). Cryptogenic stroke according to TOAST criteria is a stroke which is not due to cardiogenic embolism, small vessel disease with lacunes or large vessel disease of brain supplying arteries. In the context of secondary stroke prevention studies, cryptogenic stroke is not operationally defined. RECENT FINDINGS: The new concept of "embolic stroke of undetermined source" (ESUS) provides an operational definition. ESUS is diagnosed as a non-lacunar stroke on cerebral imaging and exclusion of large vessel atherosclerosis by CTA, MRA or ultrasound. Cardiogenic embolism is made less likely by ECG monitoring and echocardiography. At present, aspirin is used for secondary stroke prevention in patients with cryptogenic stroke. Based on the construct that ESUS might be caused by undetected atrial fibrillation or other embolic mechanisms, ongoing randomised secondary stroke prevention trials are comparing non-vitamin K oral anticoagulants (NOACs) with aspirin.