Chronic Obstructive Pulmonary Disease (COPD) is a chronic incurable respiratory condition. Part of the condition is increasing lung damage which reduces the passage of oxygen from the lungs into the bloodstream. This means that some people with COPD have abnormally low levels of oxygen in their arterial blood (hypoxaemia). Increasing the percentage of inspired oxygen has been shown to reduce mortality in people with severe hypoxaemia. Ambulatory oxygen (AO) has only been widely available for prescription in the UK since 2006, when the assessment and prescription of oxygen changed to a specialist service. But adherence to AO at home is still reported as poor. This thesis sets out to explore the patients’ perceptions of their AO system and how those perceptions may influence the use of the intervention. This body of work is divided into two phases. An initial qualitative grounded theory phase which interviewed 27 partcipants in their own home. This sought to uncover patients’ perceptions of their AO, and how they used it. This qualitative phase uncovered that patients: found their AO system too heavy to carry, used their carers extensively to carry and manage their systems, were embarrassed to use AO in public, and could not recall instructions on specific use of AO. A second quantitative phase sought to develop a questionnaire to discover if the perceptions uncovered in the qualitative study were held by a different cohort of similar participants. Five people were recruited to a cognitive interviewing study which was used to develop the questionnaire based on the findings from the qualitative phase results. A further 13 people were recruited to a pilot study to test the developed questionnaire in a different cohort of people with COPD. This quantitative phase recorded that the perceptions held by the qualitative participants were also shared by those in the quantitative phase. This thesis therefore describes a mixed methods study, looking at patients’ perceptions and use of AO.