Although the majority of mechanically-ventilated patients can be rapidly transferred to spontaneous breathing, a substantial minority of them develop difficulties during weaning attempts, prolonging the length of hospital stay and increasing the risk of complications. Inversely, excessively early weaning and extubation increase the morbidity of ventilated patients. It is, therefore, essential to be able to determine the ideal time of weaning for each patient, more particularly for those likely to develop difficulties. Various clinical and laboratory criteria of weaning, or, more precisely, predictive factors of weaning, may therefore help the intensive care physician to precisely evaluate the suitability of disconnecting a patient from a ventilator. Many of the classical criteria used are frequently inaccurate in predicting the outcome of weaning, and new criteria have been described and designed to be more objective and discriminant in predicting the suitability of weaning. The authors make a critical review and analyse the limitations of these different criteria.