The purpose of this study is to qualitatively explore how physical activity engagement is initiated and maintained in adult patients with congenital heart disease (ACHD). The rising number of aging ACHD patients means they could need more healthcare resources for lifelong treatments to maintain their health. One cost-efficient and natural way to maintain the health of ACHD patients is through physical activity participation. Unfortunately, though, ACHD patients tend to be sedentary, and research has yet to adequately explore the factors influencing physical activity behaviour within this population. To gain a better understanding on this topic, this study combines the transtheoretical model and social ecological model to uncover factors influencing physical activity behaviour in ACHD patients who are at different levels of motivation. This study utilizes a qualitative research design: over six months, 27 participants took part in three individual semi-structured interviews for a total of 78 interviews (three participants did not complete all three interviews). The study revealed that several factors influenced physical activity behaviour in ACHD patients at different stages of motivation. In addition, findings on the characteristics of participants in each stage, meanings of physical activity and strategies for promoting physical activity for ACHD patients are also presented, as well as practical findings (i.e., physical activity program for ACHD patients). This thesis is important for healthcare professionals as it could facilitate the development of stage-specific interventions designed to increase physical activity behaviour in ACHD patients.