The general issue of weaning can be viewed as composed of three different groups of patients. First, simple or easy weaning, represents 60% to 70% of patients whose first trial of spontaneous breathing is successful. The main objective of the weaning process is to detect weaning readiness as early as possible, which is best achieved using a systematic approach. The percentage of patients in this group in a given intensive care unit represents the pretest probability of weaning. A second group is made of patients who experience failure of the first spontaneous breathing trial and in whom up to 7 days from the first trial may be required to achieve weaning. This group represents 20% to 25% of patients who undergo weaning from mechanical ventilation. Muscle weakness contributes to the prolongation of weaning in many of these patients. The last group is made of patients who are characterized by a prolonged or very difficult weaning process (about 5% to 15% of patients undergoing weaning). Muscle weakness is likely to be a major contributing factor. Early use of spontaneous breathing, well-controlled use of sedation, and early mobilization may help in reducing muscle weakness and hasten the weaning process. The postextubation period may be particularly at risk in these patients. More research is needed to guide clinicians regarding the best ventilatory management.