Transitioning a patient from ventilator support to spontaneous (unassisted) breathing is defined as weaning. Despite the fact that it is an important step in recovering from respiratory failure, the weaning process was not clearly defined until recently. Based on the results from two large multicenter observational studies (the WIND and WEAN SAFE studies), weaning outcomes according to weaning classification and risk factors for weaning failure are described. Patients could be classified in the following weaning groups: patients who never entered the weaning phase, patients who could be extubated within a day (short), within 7 days (intermediate) or 7 days (prolonged) after the first separation attempt, or patients who had a failed separation attempt and could not be extubated (failure). Important factors increasing weaning delays and as such associated with weaning failure have been identified. Among these factors, sedation plays an important role and is potentially modifiable.