Heliox is a low density gas mixture of helium and oxygen commonly used in deep diving (> 6 ATM). This mixture has been also used for clinical purposes, particularly in the critical care setting. Due to of its physical proprieties, Heliox breathing reduces air flow resistances within the bronchial tree; in patients with obstructive lung diseases Heliox may also reduce the work of breathing and improve pulmonary gas exchange efficiency. Beneficial effects have been documented in severe asthma attacks and in patients with chronic obstructive pulmonary disease. A reduction in WOB during mechanical ventilation and an increase in exercise endurance capacity have also been described in COPD. Heliox has been also used in the treatment of upper airways obstruction, bronchiolitis and bronchopulmonary dysplasia. Despite the encouraging results, Heliox use in routine practice remains controversial because of technical implications and high costs.