Recent research has begun to highlight the potential for gratitude (trait, state, and interventions) to buffer against the negative effects of cardiovascular responses to stress. This thesis extends this research by examining the impact of trait gratitude on the risk of suffering acute myocardial infarction, the effects of state gratitude on cardiovascular reactivity and recovery, and the efficacy of a gratitude intervention in modulating cardiovascular reactivity. It presents three empirical studies addressing these objectives, thus providing new insights into the intersection of positive psychology and cardiovascular health. Study One’s key novel finding is that there is an indirect inverse relationship between trait gratitude and the risk of acute myocardial infarction, mediated by heart rate reactivity. Study Two is the first study to suggest that state gratitude impacts both cardiovascular reactivity and recovery, further extending research on the relationship between gratitude and cardiovascular stress responses. Study Three makes use of a randomised controlled trial design, showing that a brief intervention can reduce cardiovascular stress reactivity. These studies constitute novel explorations of the longitudinal relationship between trait gratitude and risk of acute myocardial infarction, the protective relationship between state gratitude and cardiovascular recovery, and it also provides the first study to demonstrate the influence of a brief gratitude intervention on cardiovascular reactivity. These empirical analyses suggest a modest but significant relationship between gratitude and cardiovascular stress responses that is ultimately cardio-protective.