Objectives: Depression is often unrecognised or sub-optimally treated in primary care. This has led to research exploring the barriers and enablers to effective recognition but little is known about health care professionals' (HCPs) beliefs or personal illness models about depression in patients with long term conditions (LTCs), the presence of which may affect recognition and management. Using Leventhal's Common Sense or Self-regulatory Model this thesis aimed to: explore HCPs' illness representations and management in people with LTCs and depression; to understand the role of personal models and perceived barriers to depression recognition and management; and to address them in a theory-based online training intervention.Methods: This thesis was undertaken in three stages; a scoping review with narrative synthesis was conducted to explore the role of HCPs' personal illness models of depression, a qualitative study using semi-structured interviews with 16 HCPs to investigate their illness beliefs about depression in patients with LTCs and finally, the development and feasibility assessment of a theory-based online training intervention to target HCPs' attitudes, beliefs and self-efficacy. During this stage a new measure of HCPs' beliefs and attitudes towards depression in patients with LTCs was developed as no appropriate measures were currently available to capture HCPs' personal illness models of depression as a comorbid condition.Results: Twenty-seven papers were included in a mixed method scoping review. The review concluded that HCPs mainly normalised depression but lacked a complete conceptualisation of depression in primary care. HCPs reported time constraints and lack of skills as important barriers to recognising of depression in primary care. Negative attitudes towards depression and lack of confidence to recognise and manage depression were also commonly reported. In the qualitative study, the main themes were; 1) Recognition of depression in people with LTCs is complex (unclear illness identity) 2) Attitudes towards recognition and management of depression in people with LTCs act as either barriers and enablers to depression management 3) The necessary level of condition-related knowledge and understanding of depression in the context of a LTC 4) Controllability of depression in people with LTCs. The findings suggested that HCPs' illness beliefs about depression in people with LTC varied in crucial ways with some participants prioritising the management of the LTC or expecting patients with diabetes or CHD to diagnose depression themselves due to time constraints. Some HCPs also reported simplistic views of causation which appeared to impact on their decisions whether or not to detect depression in people with LTCs. The findings of the qualitative study were used to inform the content of a novel online training intervention using illustrations to facilitate engagement. Thirty one HCPs were recruited and 15 completed the evaluation. The results of the feasibility study suggested that the method, context and mode of intervention was feasible but the lack of validated measures and the small sample size hinder conclusions about changes in participants' illness beliefs, intentions, self-efficacy and perceived barriers towards depression.Comments: The scoping review study provided new insights into why HCPs may not engage with detection of depression but only a limited amount of research has explored HCPs' beliefs about depression in patients with LTCs. The qualitative study addressed this and contributes new knowledge about the way HCPs conceptualise depression in patients with LTC and was used to design an online intervention to improve the management and recognition of depression in these patients. Further research to develop this intervention and evaluate it on a larger scale is needed.