Background: Spending long periods of time sedentary is associated with poorer health outcomes, yet adults with long term conditions and depression have high rates of sedentariness. Clinical guidelines for health are not clear on how to support adults with these conditions to reduce sedentary behaviours and no intervention targeting sedentary behaviours in this population group has been developed in the UK. This PhD study aimed to systematically develop a health behaviour change intervention to reduce sedentary behaviour and substitute it with light physical activity. Methods: The PhD study was informed by three complementary frameworks of health behaviour change interventions: the MRC framework for developing and evaluating complex interventions, the Behaviour Change Wheel, and the Person Based Approach. The intervention was developed by synthesising 1) a qualitative study to understand day to day barriers and enablers of physical activity and sedentary behaviours for this population group, 2) a systematic review of quantitative studies to examine the effectiveness of interventions and components associated with better effectiveness 3) patient and public involvement. A mixed-methods feasibility study explored the intervention's acceptability using the theoretical framework of acceptability, and its mechanisms of action. Results: In the qualitative study, the findings suggested that the behaviour change intervention should target screen-based sedentary behaviours and substitute these behaviours with light physical activity, and that sedentary and physical activity behaviours are underpinned by factors within the physical environment, the social environment, experiences of pain and fatigue, and symptoms of depression and anxiety. In the systematic review, no evidence was identified for interventions to reduce sedentary behaviours, and five out of ten randomised controlled trials found small to medium effect sizes on increasing duration and frequency of physical activity in the short and long term. Synthesising the evidence base with patient and public involvement, the intervention consisted of behaviour change techniques: unspecified, practical, emotional social support, action planning, goal setting behaviour, self-monitoring of behaviour, commitment, self-talk, information about health consequences, and mental rehearsal of successful performance delivered through a video, a booklet, and an online peer support group on Facebook. The feasibility study found that the video and booklet were acceptable however Facebook was not an appropriate platform for the support group. Participants linked the intervention with doing more household tasks, trips to the supermarket, and spending time with friends. Conclusions: This thesis is the first to report on a development of an intervention targeting sedentary behaviour in adults with long term conditions and depression. Future research using a randomised controlled design is needed to calculate effect sizes of the intervention on time spent sedentary.