This trial will enroll approximately 6,000 patients with recent embolic stroke of unknown source (ESUS). Patients will be randomized to dabigatran or acetylsalicyclic acid (ASA) (1:1 ratio) and have visits every three months. The study doctor may prescribe blinded concomitant ASA for pts with coronary artery disease but this is not mandatory. All Adverse Events (AEs), Serious Adverse Events (SAEs), outcome events will be recorded. The trial will conclude when the required number of stroke events are positively adjudicated which is estimated to take 3 years (including 2.5 years of enrollment).