Weaning from mechanical ventilation is difficult when the respiratory muscles are unable, because of weakness or fatigue, to assume the entire work of breathing. Partial ventilatory support may then be used to rest the respiratory muscles. However, the degree of muscle rest achieved varies according to the ventilatory mode. Inspiratory muscle electromyographic activity is only moderately reduced with assisted ventilation (AMV) and synchronized intermittent ventilation (SIMV). On the other hand, inspiratory pressure support (IPS) is able to markedly relieve the inspiratory muscles. With noninvasive techniques, muscle rest is better achieved by positive pressure than by negative pressure ventilation. The opposite option, training of the respiratory muscles, has also been found to facilitate difficult weaning. The role of resting and overloading the respiratory muscles in the management of difficult weaning remains to be established.