In addition to the initial lesions related to the brain injury, the final outcome depends also largely on the secondary ischaemic lesions. These lesions result from hypoperfusion whatever the mechanisms. It is then essential to detect early the conditions at risk of ischaemia, to be treated aggressively since the initial care. The multimodal monitoring is the only way to diagnose these conditions, to provide information about the mechanisms, and then to better adapt the treatment. Such an early detection helps to limit the consequences of the initial trauma, and may improve outcome.