Cryptogenic stroke (CS) is defined as cerebral ischemia of obscure or unknown origin. The cause of CS remains undetermined because the event is transitory or reversible, investigations did not look for all possible causes, or because some causes truly remain unknown. One third of the ischemic strokes is cryptogenic. CS is more frequent in younger than older patients and most frequently due to cardiac embolism, followed by vasculopathy, and coagulopathy. The most frequent causes of cardiac embolism include paradoxical embolism from upstream veins via a patent foramen ovale (PFO), paroxysmal atrial-fibrillation, valvular heart-disease, and atrial septal aneurysm. The most frequent vascular causes of CS are complex aortic plaques and Fabry's disease. Diagnostic work-up for CS includes transesophageal echocardiography, long-term ECG-recordings, CT-/MR-angiography of the aorta, transcranial Doppler-sonography, imaging for venous thrombosis in case of paradoxical embolism, and blood chemical investigations and coagulation tests. Recurrence rate and prognosis of CS is under debate. Primary and secondary stroke prevention in CS is not at variance from stroke of known cause. If the cause of CS can be identified, appropriate treatment is indicated. A PFO requires antiplatelet medication, OAC if there are other indications for OAC, and closure in case of recurrent CS under OAC.