Non-adherence to inhaled corticosteroids (ICS) is a key barrier in asthma management. However, few studies have explored patterns of non-adherence and the reasons for variations in adherence in young people with problematic asthma. The aim of this thesis is to explore the potentially modifiable determinants of non-adherence in young people with problematic severe asthma in a tertiary care setting. This PhD comprises a systematic review of interventions to improve adherence in children with asthma; an analysis of patterns of non-adherence; a qualitative study of patients with poor adherence; and an adaptation study of the Beliefs About Medicine Questionnaire (BMQ). Each of these informs identification of interventions to improve adherence. The review found that current interventions have limited effectiveness, with only half of the included trials able to improve ICS adherence (9/18). More complex interventions, tailored to the patient, which addressed both perceptions and practical aspects of non-adherence were more likely to be effective. Secondary analysis of electronic adherence data from this population (n=93) identified adherence patterns which have implications for intervention development. The interview study (n=20) identified perceptual determinants (e.g. poor understanding of asthma and ICS) and practical determinants (e.g. no routine and forgetfulness) of non-adherence. These findings informed an adaption of the BMQ to identify beliefs underlying treatment non-adherence in this population; initial piloting (n=30) revealed high overall internal reliability but further research is needed to validate the questionnaire. This PhD highlights the need for a tailored intervention for non-adherent young people with problematic asthma which addresses perceptual and practical barriers to adherence. The PhD identified new barriers to adherence including key differences between adults and young children. A belief-based questionnaire could be used to identify modifiable beliefs for inclusion in a tailored intervention addressing both perceptual and practical barriers for adherence to ICS.