For over 70 years, helium-oxygen mixture (heliox) has been promoted as adjunctive therapy to overcome airflow-obstructive disorders and lesions. In the past 2 decades heliox has gained widespread support in many pediatric emergency departments and intensive care units, in treatment of infants and children with both upper and lower airway obstruction. Because heliox is less dense than air or oxygen, it provides more laminar flow in obstructed airways, and it is purported to reduce work of breathing, respiratory distress, and postextubation stridor. Clinical evidence of the effectiveness of heliox in pediatric patients with airflow obstruction is relatively sparse and appears in the literature primarily as case presentations, case series, and small, uncontrolled studies. This article reviews the rationale and methods for heliox treatment of children with asthma, airway obstruction, bronchiolitis, and croup.