This abstract details the development phase of the formative research process outlined by the Medical Research Council, used to develop a theoretically informed Android App, named MedFit, to enhance disease self-management and quality of life in adults with cardiovascular disease (CVD). The overall aim of the app is to increase physical activity minutes of adults with CVD. A key part of the development phase, which is the focus of this abstract, is the co-design and user validation of the MedFit app.