The polyneuropathy and myopathy of the critical patient are frequent complications in the ICU that are characterized by generalized muscular weakness including the limitation in the weaning of the mechanical ventilation, prolonging the hospital stay and the risk of other complications. They cause chronic disability in survivors of critical illness due to functional and structural changes in nerve and muscle fibers resulting in axonal nerve degeneration, loss of muscle myosin, cellular hypoxia and muscle necrosis, which leads to the electrical inexcitability of nerves and muscles, with reversible muscle weakness. The diagnosis is made through the clinic, electrophysiological studies and muscle biopsy. Metabolic control and risk factors would reduce the severity of complications and early rehabilitation in the unit could accelerate functional recovery and patient independence. Abbreviations: ICU, intensive care unit; PPC, critical patient polyneuropathy; MPC, critical patient myopathy.