This thesis investigates the relationship of both trait and state explanations of Mental Toughness (MT) upon a range of behavioural and psychophysiological outcome variables that should relate to the construct of MT (e.g., performance, heart rate, muscle activity, kinematic movement, and cortisol). Chapter 1 presents a holistic overview of the strengths and limitations of research in MT and offers some novel approaches that could advance knowledge in this area. The introduction briefly explains different concepts that relate to the construct of MT. The strengths and limitations of trait (personality) and state (self-report) perspectives of MT are reviewed. Finally, future outcome variables that should be theoretically related to MT that have yet to be fully explored are discussed. This discussion sets out in detail, the purpose of the thesis. Chapter 2 aimed to advance previous research findings where personality traits (i.e., low reward and high punishment sensitivities) have been shown to predict Mentally Tough behaviour (MTb) and performance outcomes under pressure (e.g., Beattie, Alqallaf, & Hardy, 2017; Hardy, Bell, & Beattie, 2014). As suggested in the research overview from Chapter 1, these individuals may demonstrate unique psychophysiological response to stress that allow them to tolerate higher levels of pressure than their less mentally tough counterparts. Therefore, we hypothesized that individuals high in punishment and low in reward sensitivities (and those high in self-report MT) would show little or no increase in heart rate, and show stable muscle activity and movement kinematics from low-stress to high-stress conditions, compared to less mentally tough individuals. The stress condition involved participants making a single putt where they could double or lose all the money they had earned up to that point. Results indicated that, when reward sensitivity was low and punishment sensitivity increased, heart rate reactivity was blunted and movement kinematics (club-head angle) were more consistent when transitioning from a low to high stress environment. However, no significant relationships were found between self-report levels of MT, psychophysiological and movement kinematic measures. Chapter 3 addressed some of the limitations from Chapter 2. Specifically, the stress manipulation was modified to provide participants with early warning of the stressor, and, therefore, more time to prepare. The stress manipulation was also intensified by removing money from participants for missed putts, and adding peer pressure by having participants complete the experiment in pairs. We also extended the psychophysiological approach from Chapter 2 by examining cortisol. Results regarding personality and heart rate differed slightly from Chapter 2. Importantly, with early warning of the stressor, personality no longer predicted heart rate reactivity, but it did predict preparatory heart rate deceleration, an index of motor preparation. Preparatory heart rate deceleration was disrupted on transition from low-stress to high-stress conditions, but when reward sensitivity was low, increasing punishment sensitivity was associated with more consistent deceleration across both low-stress and high-stress conditions. Moreover, when reward sensitivity was low, increasing punishment sensitivity was associated with less angular error (better performance). Finally, contrary to our hypothesis, cortisol increased from the high stress condition to the low stress condition. Chapter 4 draws upon studies of early versus late preparation, and prevention versus promotion focus, to account for the subtly different results across Chapter 2 and Chapter 3. In doing so, it discusses the theoretical and applied implications of the thesis. Limitations and strengths of the thesis are discussed and future research directions are proposed.