The 3rd and updated edition of this book represents a new and unique scientific reference for the medical community on how to understand rationale and applications of noninvasive mechanical ventilation (NIMV). Its aim is to establish the indications of NIMV in critically ill patients in weaning from invasive MV. Nowadays, there is a growing evidence-based medicine that recommends use of NIMV in patients after extubation or in difficult weaning patients also affected by comorbidities. This book has been conceived with the vision or providing the best resources for everyone working in ICUs even if belonging to different specialties (intensive care, anesthesiology, pneumology, emergency medicine, etc.). Considering the enormous increase of literature on this topic, authors have selected major key topics related to NIMV, excluding those with low rate of interest, have updated previous topics and have introduced new items collecting them in a practical book analyzingmajor key topics for a correct practical applications. A new gaze has been devoted to emergency medicine and prehospital applications and technical developments (new ventilation modes: neurology adaptive modes, average support mode and to the development of synchronization and patient-ventilator interaction result). A section dedicated to sleep medicine - due to the new interesting studies on NIV-CPAP adaptation studies, clinical impacts of CPAP devices and ventilatory modes representing an essentials development for a new adequate analysis - is now included. A part devoted to clinical indications based on the observation of new clinical indications in anesthesiology and pneumology in NIV as complementary technique for procedures like bronchoscopy, pre-oxygenation and difficult endotracheal intubations is also now foreseen.