The current thesis re-investigated the complex link between revised reinforcement sensitivity theory (rRST) and mentally tough behaviour and tested whether rRST could be extended to predict mentally tough athletes psychophysiological responses. The objectives of this thesis were to address two issues in Hardy, Bell, and Beattie’s (2014) study, assess whether rRST could predict the psychophysiological responses and performance of mentally tough athletes, and test the utility of two competing cardiac models to map these responses. Study 1 addressed the issue of the measurement of rRST in Hardy et al.’s study by examining the factorial validity of the six-factor Reinforcement Sensitivity Theory Personality Questionnaire (RST-PQ). Utilising a technically complex, alternative statistical approach in Bayesian structural equation modelling (BSEM), analyses supported the RST-PQ’s six-factor structure in two separate samples of student athletes. Study 2 addressed the issue of the statistical procedures used to validate the single-factor 15-item Cricket Mental Toughness Inventory (CMTI) in Hardy and colleague’s study, and re-examined their results using this measure and the RST-PQ. Results offered further for BSEM with the 15-item CMTI measurement model demonstrating a strong model-fit in a sample of male elite and sub-elite cricket coaches. Multilevel analyses also revealed unique insights into the personality of male elite and sub-elite cricketers where the mentally toughest performers elicited high avoidance motivation levels and increased approach motivation levels. Study 3 tested the hypothesis that rRST could predict mentally tough athletes psychophysiological responses and performance with an ambitious experiment in a real-world sporting context. In addition, study 3 took a novel approach to depict these responses, by examining the effectiveness of the Cardiac Autonomic Balance and Cardiac Autonomic Regulation (CAB/CAR) models. Hierarchical regression analyses showed that rRST did not significantly predict the psychophysiological responses and performance of male elite cricketers. However, partial support was provided for CAB with a significant main effect between approach motivation and CAB. Implications of this research programme have far-reaching effects for sports and other disciplines (e.g., military, medicine) where performing under pressure is crucial for success. Study 2 suggested that the personality trait of high approach and avoidance motivation levels plays a key role in successful performance. This finding combined with the validation of the RST-PQ from Study 1, means practitioners now have available a reliable measure to accurately assess personality. Additionally, the study 2 findings provide a template for tailoring training environments to foster and nurture success. Study 3, whilst contradicting initial predictions, implies that to promote optimal performance, it is important to create the right pressure levels in training. In conclusion, this thesis progressed Hardy et al.’s (2014) study by validating a contemporary rRST measure, enhanced their CMTI using BSEM, and updated the rRST-mentally tough behaviour relationship. Lastly, scant support was offered for a link existing between rRST and psychophysiological responses and the ability of the CAB/CAR models to map these responses was inconsistent with more research needed.