The principal role of the respiratory system is to permit effi cient exchange of respiratory gases(O2 and CO2) with the environment. The respiratory system is unique in that it is constantlyexposed to a barrage of foreign substances from both the internal environment (at any one pointin time, approximately one-half of the cardiac output is received by the lungs) and the externalenvironment (with each breath, the respiratory tract is exposed to pollens, viruses, bacteria,smoke, etc). According to the Centers for Disease Control and Prevention, diseases of the respiratorysystem were the seventh and eighth leading causes of deaths in children aged 1 to 19 yearsin 2003 [1]. Dr. George A. Gregory, one of the founding fathers of pediatric critical care medicine,once estimated that acute respiratory failure accounts for nearly 50% of all admissions tothe pediatric intensive care unit (PICU) [2]. Just as important are the many diseases that affectthe respiratory system that are not associated with acute respiratory failure, but neverthelessconstitute a major portion of the practice of pediatric critical care medicine, some of whichaccount for signifi cant morbidity and mortality [3]. Once again, we would like to dedicate thistextbook to our families and to the physicians and nurses who provide steadfast care every dayin pediatric intensive care units across the globe.