Noninvasive mechanical ventilation (NIMV) is an effective technique that can avert side effects and complications associated with endotracheal intubation. The tolerance of NIMV is crucial for its success. Mask intolerance because of pain or discomfort, or claustrophobia, may lead the patient to refuse ongoing NIMV prompting its discontinuation. After considering all factors known to improve adaptation of the patients to NIMV, the implementation of sedation-based strategy to rescue patients with poor cooperation and/or adaptation to NIMV is remaining attaractive to enlarge its rate of success. Pilot studies suggest that continuous infusion of a single different sedative and analgesic agent titrated to obtain "conscious sedation" may decrease patient discomfort, with no significant effects on respiratory drive, respiratory pattern, or hemodynamic; in addition, gas exchange improve under NIMV plus sedation. Despite these encouraging findings, the level of the evidence in favor of a large application of sedation during NIMV is still limited and further larger and controlled trials are needed to clarify the indications of sedation during NIMV and better select the patients who are mostly likely to benefit from this practice. In this review, indications of sedation during NIMV, the timing, sedative drugs, the level of sedation as well as clinical studies on this subject have been summarized.