ICU-acquired weakness (ICUAW) is often found in critically ill patients and is characterised by a generalised, symmetric weakness, affecting limb (proximal more than distal) and respiratory muscles. It is a diagnosis of exclusion. ICUAW is associated with increased morbidity and mortality, and decreased quality of life. The aetiology is not completely understood, and there is no established treatment or prevention yet. The most important risk factors are high disease severity, multiorgan failure, systemic inflammation and prolonged immobilisation. Individual needs for rehabilitation must be defined, as argued in this review.