Stays in intensive care units are commonly accompanied by muscle weakness. This article reviews the basis for these changes and provides guidance on how to prevent, diagnose, and treat this condition. Weakness acquired in the intensive care unit (ICU) is caused by many different pathophysiological mechanisms that are not mutually exclusive. This is not surprising, given the diverse diseases that precipitate critical illness, the drugs used during its management, and the consequences of protracted immobility. Nonetheless, conceptualization of this entity is valuable, since weakness in survivors of critical illness is common and is associated with long-standing consequences that dramatically affect recovery. Moreover, as survival rates among patients in the ICU increase, ICU-acquired weakness will have increasing relevance for care providers outside the ICU. This article provides an overview of the condition . . .