Neuromuscular disorders alone can account for up to 62% of the cases of ventilatory dependency in intensive care unit (ICU) patients with no other apparent causes of failure to wean, and they may be a contributing factor in up to 86% of these patients (Spitzer et al. 1992). In addition, neuromuscular abnormalities can be detected in up to 95% of patients who stay in the ICU for a period of more than 7 days (Coakley et al. 1993). Among the neuromuscular disorders, critical illness polyneuropathy (CIP) and critical illness myopathy (CIM) have become two of the most common underlying diagnoses associated with an otherwise unexplained need for prolonged ventilatory support. Although in the earliest reports on CIP and CIM most patients had sepsis and multiple organ failure or were given neuromuscular blocking agents or high doses of corticosteroids, respectively, it is now well accepted that the spectrum of primary medical diseases in these patients is much broader than initially suspected and that the above-mentioned pharmacological precedent is not a constant feature (Berek et al. 1996; Coakley et al. 1993). In this chapter, we describe the epidemiological, clinical, electrophysiological, and pathological characteristics of both entities and summarize the relevant findings related to their pathogenesis.