This thesis is written as a collection of research papers through which the role of emotional expression in performance and health was examined. Chapter 1 reviews specific research literature related to emotional expression in performance and health, and outlines the avenues of research investigated within the thesis. Chapter 2 contains three experimental studies that explored the influence of the emotions of happiness, hope and anger on acute cognitive and physical performance. The results of Experiment 1 revealed that anger enhanced physical performance, but cognitive performance was not enhanced by happiness or anger. Experiment 2 examined the role of effort in the emotion-performance relationship and found that hope and anger were related to increased investment of effort on the cognitive task; however only hope was associated with improvements in reaction times. Experiment 3 investigated the moderating influence of personality on the anger-physical performance relationship. In a replication of the physical task used in Experiment 1, extraverts' anger-derived performance gains were found to be greater than those of introverts'. Chapter 3 extended the findings of Chapter 2 by examining the role of anger-related individual differences and physiological reactivity as potential moderators of the anger-performance relationship. The results revealed that trait anger and the anger coping style of anger-out were associated with anger-derived performance enhancement. The anger coping style of anger-in was found to negatively influence the trait anger-performance relationship. Further, anger was associated with increased physiological arousal as increased salivary alpha-amylase was associated with anger; however, physiological reactivity was not related to anger-derived performance enhancement. Chapter 4 describes a six-month intervention-based study that examined a number of theoretical models that have been proposed to explain the mechanisms that underpin the beneficial effects of written emotional disclosure in fibromyalgia. The theoretical models of inhibition and exposure were found to have no association with specific health outcome variables. However, the theoretical model of cognitive adaptation was associated with improved health on a number of the outcome measures assessing the central features of fibromyalgia (i.e. , physical impairment, pain, and stiffness). Chapter 5 discusses the findings arising from the research chapters, presents the central theoretical and applied implications, identifies the strengths and limitations of the research programme, and provides suggestions for future research.