Over the last twenty years, increasing numbers of critically ill, mechanically ventilated patients who develop acute profound muscle weakness have been described. These acute weakness syndromes have not been well understood and they have been given many names including: acute steroid myopathy, acute quadriplegic myopathy, the floppy person syndrome, critical illness polyneuropathy, critical illness polyneuromyopathy, and prolonged neurogenic weakness. Many of these "syndromes" either overlap or represent the same disease process in different patients. Many have been incompletely diagnosed. During this review it became evident that the acute weakness syndromes currently recognized in critically ill patients could be categorized into four major groups: myopathy, neuromuscular junction abnormalities, neuropathy and polyneuromyopathy. Each had different possible aetiologies. "Myopathy" includes acute necrotizing myopathy and disuse atrophy. Neuromuscular junction abnormalities are subdivided into myasthenia-like syndromes and prolonged neuromuscular blockade. Neuropathies are divided into critical illness polyneuropathy and acute motor neuropathy. The anterior horn cell injury in Hopkins syndrome should also be considered in this group. Polyneuromyopathies include various combinations of neuropathy and myopathy in the same patients.