The mechanical properties of the respiratory system such as resistance, elastance and mechanical work of breathing are rarely measured directly but are inferred from the effect of respiratory disease on maximal lung volumes and flows. Although such tests have proved very useful, they have shortcomings, e.g. changes in lung volumes are poor at detecting progression in interstitial lung disease and correlate only weakly with changes in functional capacity achieved post-bronchodilator in patients with airways obstruction. The direct measurement of mechanical properties is of interest as they have an obvious physical interpretation but their usefulness has as yet not been systematically tested. Resistance aside, their measurement is rarely performed as it is invasive, requiring either a sedated patient on controlled ventilation to abolish spontaneous respiratory muscle activity or measurement of oesophageal and gastric pressures. The aim of this thesis was to explore the feasibility and potential clinical value of non-invasive measurements of respiratory mechanics and work of breathing. The work is presented in three sections. Firstly, conventional methods for measuring resistance, elastance and mechanical work of breathing were reviewed and the methods for the non-invasive approaches to be used were described in detail. The results from the non-invasive methods were then validated by comparison with conventional techniques in both ventilated patients and in subjects in the pulmonary function laboratory where oesophagal and gastric manometry were performed. Finally, the non-invasive methods were evaluated in three clinical scenarios: bronchodilator reversibility testing, assessment of progression in interstitial lung disease, and monitoring recovery from exacerbation of chronic obstructive pulmonary disease.