The 2nd edition of this book aims to underline how inadequate humidification of inspired gases can be the cause of a variety of serious problems and, thus, it brings new results and trends in humidification, updates about technological analyses in equipment s ventilator modes and again the impact of humidification in complementary therapies such airway secretions in mechanical ventilated patients. These aspects are analysed in critically ill patients requiring various options of ventilatory approach (i.e. invasive, noninvasive, nasal high flow oxygen). The book starts with an exhaustive description of the pathophysiology of humidification in critically ill, and continues analyzing the impact of mechanical ventilation modalities (high-flow oxygen therapy, noninvasive mechanical ventilation, invasive mechanical ventilation, etc.), monitoring prevention of complications related to inadequate humidification.