Manual lung hyperinflation (MH) is one of a number of techniques which are employed by the physiotherapist in the critical care setting. The technique was first described with physiotherapy 30 yrs ago and commonly involves a slow, deep inspiration, inspiratory pause and fast unobstructed expiration. The use of MH varies between and within countries. It is commonly employed by physiotherapists to assist in the removal of secretions and re-expand areas of atelectasis. Despite the popularity of the technique, research examining its efficacy is conflicting, especially the effect of MH on cardiovascular parameters. Recent studies examining mucociliary transport in intubated and ventilated patients have shown impaired clearance of secretions, but research evaluating the role of MH specifically in airway clearance is scant. The use of the additional physiotherapy techniques, gravity assisted drainage and chest wall vibrations, may enhance the efficacy of MH in promoting airway clearance, but further research is necessary. Controversy exists regarding the safety and effectiveness of application of manual lung hyperinflation in intubated patients. Clearly, more randomized controlled studies are necessary in order to provide a sound scientific rationale for the application of manual lung hyperinflation in the treatment of critically ill patients.