This book gathered quantitative evidences of clinical efficacy when using high-flow nasal cannula (HFNC) in the intensive care units in hospitals. We collected data from published prospective studies that compared the efficacy between patients who received HFNC and patients who received standard oxygen therapy or noninvasive ventilation (NIV). We evaluated the efficacy of HFNC on physiological and clinical outcomes including vital signs, oxygenation and hemodynamics, patient discomfort, hospitalization length of stay and mortality and adverse events. This book provided statistical analyses of existing literatures evaluating the use of HFNC for three major applications: (1) acute respiratory failure; (2) pre-oxygenation for intubation; and (3) post-extubation. Each clinical outcome was summarized in a table to compare the specific efficacy between patients who received HFNC and patients who received standard oxygen or NIV. A meta-analysis result was presented as a plot to compare the specific efficacy, for example, the incidence rate of post-extubation re-intubation, between patients who received HFNC and patients who received standard oxygen or NIV.