As the prevalence of breast cancer in the community increases, increasing numbers of women will face the psychosocial burden of this disease and its treatment. Peer support has emerged internationally as a community-based model of psychosocial care and involves the mutual sharing of experiences about breast cancer amongst those affected with this disease. Despite the psychosocial benefits reported by women who engage in this behaviour, utilisation of peer support services is estimated to be low and factors that influence its use are unclear. However, qualitative evidence suggests users hold distinct perceptions about peer support based upon these interactions involving other women with breast cancer. Accordingly, the aim of the present thesis was to better understand peer support behaviour amongst women with breast cancer by exploring the determinants of peer support service use from a social cognitive perspective.