Neuromuscular pathology in the critically ill patient develops within two settings: primary neurological diseases that require admission in the Intensive Care Medicine Unit for close monitoring or mechanical ventilation, and peripheral nervous system manifestations secondary to critical systemic diseases. The most frequent conditions in the first group are Guillain-Barré syndrome and Myasthenia Gravis, and in the second group, polyneuropathy and myopathy of the critically ill patient. The most commonly shared clinical pattern is the development of severe weakness and quadriplegia which most typical manifestation is the need for assisted ventilation and/or weaning difficulty/impossibility. Triggering factors considered are multiorgan failure and sepsis in polyneuropathy, and steroids and neuromuscular blockers in myopathy, with malnutrition, particularly hypoalbuminemia, and hyperglycemia being co-adjuvant in both conditions. Considering that neuropathic and myopathic conditions may frequently coexist, the term polyneuromyopathy of the critically ill patient has been coined. Both Guillain-Barré syndrome and polyneuropathy of the critically ill patient involve peripheral nerves, so that the differential diagnosis has to be made between both. The presenting picture is different, since the former is an acute pathology that motivates ICU admission, whereas the latter is a polyneuropathy acquired during hospitalization. In the former, involvement of the autonomous nervous system and CSF albumin-cytology dissociation are common, which do not occur in polyneuropathy. Electrophysiological studies show demyelinating signs with decreased conduction velocity and normal amplitude of motor potentials in Guillain-Barré syndrome versus normal conduction velocity and reduced amplitude of motor potentials in axonal polyneuropathy. Myasthenic crisis affects the neuromuscular junction and its diagnosis tends to be easier since in most of the cases a previous diagnosis of myasthenia gravis exists. Muscle weakness increases during repeated activity (muscle fatigue) and improves on resting. Diagnostic confirmation is done by means of edrophonium test and by repeated nerve stimulation, which leads to a rapid decrease by 10-15% of the amplitude of evoked responses. Myopathy of the critically ill patient involves the muscle and provokes a generalized weakness with quadriplegia, very similar to that from polyneuropathy, which prevents or delays weaning from mechanical ventilation, and which may lead to CPK and myoglobin increase in more advanced stages, together with changes in neurophysiological examination. The findings of neurophysiological examination are difficult to differentiate from those encountered in polyneuropathy, although normal sensitive action potentials and reduction of motor action potentials with direct muscle stimulation may help in the differentiation. The functional prognosis of primary muscle impairments tends to be quite good, but both polyneuropathy and myopathy resolve very slowly along weeks or months, with the possibility of an important residual deficit within two years in the most severe cases.