PURPOSE OF REVIEW: To describe the incidence, major risk factors, and the clinical, electrophysiological, and histological features of critical illness myopathy (CIM). Major pathogenetic mechanisms and long-term consequences of CIM are also reviewed. RECENT FINDINGS: CIM is frequently associated with critical illness polyneuropathy (CIP), and may have a relevant impact on patients' outcome. CIM has an earlier onset than CIP, and recovery is faster. Loss of myosin filaments on muscle biopsy is important to diagnose CIM, and has a good prognosis. Critical illness, use of steroids, and immobility concur in causing CIM. SUMMARY: A rationale diagnostic approach to CIM using clinical, electrophysiological, and muscle biopsy investigations is important to plan adequate therapy and to predict recovery.