Haemorrhagic transformation (HT) of ischaemic infarction occurs when an area of brain infarction is stained with blood products, mainly red blood cells. An abnormally permeable blood-brain barrier resulting from ischaemia of the capillary endothelium allows this extravasation of blood products. HT is part of the natural history of some forms of ischaemic infarction, especially cerebral embolism, but it can be precipitated or enhanced by therapeutic interventions used in the acute phase of ischaemic stroke. The old view of HT after cerebral embolism as a generally asymptomatic change in a tissue that is already necrotic has been challenged by observations from therapeutic thrombolysis that suggest that HT can have a negative effect on patients' outcomes. Understanding of the risk factors for and the underlying mechanisms and clinical variability of HT in the context of acute therapeutic interventions in ischaemic stroke could help in the early detection of this complication, in determining the safety of recanalisation approaches, and in setting the stage for future research into the prevention or treatment of HT in patients with acute ischaemic stroke. © 2013 Elsevier Ltd.