In about a quarter of ischaemic strokes the cause is undetermined, because the investigation is incomplete or delayed, because there are multiple causes or because the stroke is truly cryptogenic. Cryptogenic stroke can be further classified as non‐embolic or embolic. Embolic stroke of undetermined source can be due to paroxysmal atrial fibrillation, minor emboligenic cardiac conditions, atheroembolism, cancer associated and paradoxical embolism through a patent foramen ovale (PFO) or less often a pulmonary fistula. Currently, risk factor control, statins and antiplatelets are the main therapeutic measures to prevent recurrent stroke. There is no evidence to implement routine closure of PFO in patients with cryptogenic stroke. Direct anticoagulants are being evaluated in randomized controlled trials including embolic stroke of undetermined source patients. Advances in high resolution ultrasound or magnetic resonance imaging of extracranial and intracranial vessels and of the heart and prolonged heart rhythm monitoring will be instrumental techniques to identify arterial and cardiac hidden causes of stroke.