Intensive care unit–acquired weakness (ICUAW) is a substantial contributor to long-term disability in survivors of critical illness. Critical illness polyneuropathy, critical illness myopathy, and muscle atrophy from disuse contribute in various proportions to ICUAW. ICUAW is a clinical diagnosis supported by electrophysiology and newer diagnostic tests, such as muscle ultrasound. Risk factor reduction, including the aggressive treatment of sepsis and early mobilization, improves outcome. Although some patients with ICUAW experience a full recovery, for others improvement is slow and incomplete and quality of life is adversely affected. This article examines aspects of ICUAW and identifies potential areas of further study.