Noninvasive ventilation, both continuous positive airway pressure and noninvasive positive pressure ventilation, has been used increasingly for acute respiratory failure over the past several years. Noninvasive ventilation has been proven to be beneficial for some causes of acute respiratory failure, most clearly for acute exacerbations of chronic obstructive pulmonary disease, while its use in other forms of acute respiratory failure remains more controversial. In this article, the evidence for the use of noninvasive ventilation in various kinds of acute respiratory failure will be examined. Particular attention will be paid to the clinical situations commonly encountered by emergency medicine and general internal medicine clinicians. The potential dangers of noninvasive ventilation as well as some guidelines for clinical decision making when treating patients with this mode of ventilator support will also be discussed.