Background: Cardiovascular disease (CVD) is the leading cause of premature death and disability in Europe. Cardiac Rehabilitation (CR) can reduce the impact of CVD by lowering mortality and morbidity rates and promoting healthy active lifestyles. Yet adherence within CR is low. Research suggests that mHealth interventions are useful in supporting the self-management of chronic disease. The overall purpose of this research is to facilitate the development of a specially designed Android App called MedFit, which aims to enhance the likelihood of people with established CVD self-managing their disease through participation in an exercise-based rehabilitation programme. Methods: For the intervention development, the preliminary stages of the Medical Research Council’s (MRC) formative process [i) development and ii) feasibility/piloting were used. This was achieved by conducting a systematic literature review, adhering to PRISMA guidelines, to identify what behaviour change techniques (BCTs) are used in physical activity eHealth interventions for adults with CVD (study 1). Study two involved testing the feasibility and acceptability of the prototype application using five focus groups (N=26 CVD patients; average age 64 ± 8.2 years; 65 % male). The focus group script was developed using a questionnaire (N=119 CVD patients; Appendix B.3) based on the Unified Theory of Acceptance and Use of Technology 2 (UTAUT2) which identified the constructs that were the primary concerns for end users. Focus groups were transcribed verbatim and in-depth content analysis was performed. Results: Twenty-three studies were included in the systematic literature review. The average number of BCTs employed in the studies was 7.2 (range 1-14). The top three most frequently used BCTs were information about health consequences (78.3%), goal setting (behaviour) (73.9%) and self-monitoring of behaviour (47.8%) (study 1). In study 2 the focus group usability feedback included; add in a retrieve password function, play the exercise video continuously and remove leaderboard function. Four key themes were identified following content analysis, these were; support, app as a mentor/guide, translation of activity from gym to home and technology knowledge gap. This feedback was translated into feasible technical improvements through close collaboration with the technical team. Conclusion: This research describes in detail the design process, first alpha-version of the App and focus group feedback used to develop a CVD mHealth intervention.