The hypothesis underlying this thesis was that abnormal, dysfunctional breathing may occur commonly in people with asthma, and when identified and treated using a breathing training programme supervised by a physiotherapist, will result in improved asthma control.  The thesis is based around four original research papers published in peer-reviewed journals.  These papers present epidemiological surveys quantifying the extent of symptoms attributable to dysfunctional breathing in adults with asthma in comparison with the non-asthmatic adult population, and randomised controlled trials investigating the effectiveness of a breathing training programme in improving asthma control. Initially, a review of the existing evidence of co-morbidity between asthma and dysfunctional breathing is presented, together with that of effectiveness of breathing training interventions.  In subsequent chapters, two epidemiological surveys are presented, showing that symptoms consistent with dysfunctional breathing were more common in the asthmatic than the non-asthmatic adult population.  Data from a pilot and a subsequent full randomised controlled trial are then presented.  These show that breathing training was associated with improved patient-reported outcomes in comparison with a control intervention of asthma education (chosen to control for the non-specific effects of professional contact and interest on a symptomatic patient). The thesis shows that in a clinical trial situation, many people with asthma can benefit from breathing training.