∙ Age ≥ 18 years, written informed consent to participate in the study Clinical diagnosis of ischemic stroke or transient ischemic attack + brain imaging to rule out hemorrhagic stroke.

• Stroke: ESUS, defined as all of the following:

‣ Stroke detected by CT (computertomography) or MRI (magnetic resonance imaging) that is not lacunar. (Lacunar is defined as a subcortical (this includes pons and midbrain) infarct in the distribution of the small, penetrating cerebral arteries whose largest dimension is ≤1.5 cm on CT or ≤2.0 cm on MRI diffusion images/\<1.5 cm on T2 weighted MR images. The following are not considered lacunes: multiple simultaneous small deep infarcts, lateral medullary infarcts, and cerebellar infarcts.)

⁃ Absence of extracranial or intracranial atherosclerosis causing ≥50 percent luminal stenosis of the artery supplying the area of ischemia. Patients must undergo vascular imaging of the extracranial and intracranial vessels using either catheter angiography, CT angiogram (CTA), MR angiogram (MRA), or ultrasound, as considered appropriate by the treating physician and local principal investigator.

⁃ No major-risk cardioembolic source of embolism, including intracardiac thrombus, mechanical prosthetic cardiac valve, atrial myxoma or other cardiac tumors, mitral stenosis, myocardial infarction within the last 4 weeks, left ventricular ejection fraction \<30 percent, valvular vegetations, or infective endocarditis).

⁃ No other specific cause of stroke identified, such as arteritis, dissection, migraine, vasospasm, drug abuse, or hypercoagulability. Special testing, such as toxicological screens, serological testing for syphilis, and tests for hypercoagulability, will be performed at the discretion of the treating physician and local principal investigator, if needed.

• TIA: Patients fulfilling all above criteria and diagnostic work-up, except the detection of ischemic lesions by CT or MRI is optional and clinical symptoms last \< 1 hours.

∙ All patients must undergo electrocardiogram, transthoracic or transesophageal echocardiography (TTE or TEE) and at least 24 hours of cardiac rhythm monitoring (Holter monitor or telemetry or equivalent).

∙ Patent foramen ovale is not an exclusion criterion. Planned or existing implantation of an implantable cardiac monitor or cardiac pacemaker is not an exclusion criterion.