Non-invasive ventilation (NIV) is an effective technique that can avert side effects and complications associated with endotracheal intubation. NIV is primarily used to avert the need for endotracheal intubation in patients with early stage acute respiratory failure (ARF), and to prevent postextubation respiratory failure in patients considered to be at risk. It can also be used as an alternative to invasive ventilation at a more advanced stage of ARF or to facilitate the process of weaning from mechanical ventilation. The success of NIV relies on several factors including the type and severity of ARF, the underlying disease, the timing, the location of treatment, and the experience of the team. In this review article, we analyze, compare, and discuss the results of studies in which NIV was applied in different pathologies and with different timing during the evolution of ARF.