The thesis contains four chapters which attempt to extend our understanding of mental toughness. The thesis focuses predominantly on sporting environments, which can be very stressful and often require athletes to perform under intense pressure; as such it provides a useful context to examine mental toughness. Using Reinforcement Sensitivity Theory (RST; McNaughton & Gray, 2000) as a theoretical framework, the thesis contains five empirical studies (organised into two chapters for the purpose of publication), which attempt to apply the principles of RST to understand how cricketers maintain or enhance their performance under pressure. The thesis is not a test of Reinforcement Sensitivity but an application of its principles in a novel environment. Chapter 1 critically reviews the research on mental toughness and highlights a number of theoretical and empirical limitations which need to be resolved. Most notably, these include: (i) circuitous and somewhat confusing definitions; (ii) only modest attempts to draw upon relevant theory to inform a priori hypothesis testing; (iii) a lack of valid measurement tools; and (iv) limited experimental studies that focus on the development of mental toughness. The chapter finishes by proposing a neuro-cognitive explanation of mentally tough behaviour based on the tenets of the revised Reinforcement Sensitivity Theory (McNaughton & Gray, 2000). Chapter 2 contains four separate studies. The first two studies were concerned with the development of a valid, informant-rated, questionnaire to measure mental toughness. The final two studies explored the interactive relationship between reinforcement sensitivities and mental toughness. The findings of the studies suggested that the relationship between RST and mental toughness is a somewhat complex one in that cricketers rated as mentally tough by their coaches tended to be sensitive to punishment cues and insensitive to reward cues. In contrast, cricketers 3 rated as low in mental toughness by their coaches tended to be sensitive to punishment cues and reward cues. These results are discussed and explained in terms of threat detection, behavioural inhibition and decision making accuracy. Chapter 3 depicts a quasi-experimental, longitudinal intervention study. The study reports on the design, delivery and evaluation of a theoretically grounded mental toughness training program for youth aged Academy cricketers. The intervention was designed to expose cricketers to punishment conditioned stimuli in the training environment and to equip them with effective coping skills to manage threat. The results are discussed in terms of the theoretical and applied implications of using punishment to alter behaviour. Chapter 4 concludes the thesis. More specifically, the chapter provides a summary and integrated discussion of the thesis findings, implications from both theoretical and applied perspectives, methodological and conceptual limitations and avenues for future research.