This thesis examines some of the measurement, conceptual, predictive, and intervention issues surrounding mental toughness in military training. Chapter 1 briefly reviews the research literature on mental toughness as it relates to the above issues. This review identifies questions worthy of future investigation; specifically conceptualisation and measurement issues relating to mental toughness and issues relating to the development of mental toughness, particularly in the military environment. In chapter 2, three studies were conducted to develop and validate a robust and psychometrically reliable informant-based measure of mental toughness for use in military training environments, utilizing a total of 645 infantry recruits from the Infantry Training Centre (ITC) at Catterick Garrison. Study 1 focused on item generation and identifying relevant key stressors from a range of different stressors experienced by recruits during infantry basic training to form the basis of the measure, followed by a test of the structural integrity of the resulting measure. Study 2 examined the concurrent validity, predictive validity, and test-retest reliability of the measure. A further study (Study 3) was conducted with a sample of more specialised infantry recruits to confirm the predictive validity of the measure. Overall, the Military Training Mental Toughness Inventory (MTMTI) was found to possess sound psychometric properties and structural validity, good test-retest reliability and concurrent validity, and predicted performance in two different training contexts with two separate samples. Chapter 3 deals with the somewhat controversial topic of using punishment to develop mental toughness and enhance individual performance under pressure. Specifically, the study examined the interacting effects of contingent punishment and three supportive transformational leadership behaviours on mental toughness and performance under pressure. The leadership transformational behaviours were: (1) individual consideration, (2) fosters an acceptance of group goals and, (3) inspirational motivation (c.f., Hardy & Arthur, 2010). A total of 808 recruits from ITC took part in two studies. Study 1 explored the interactive effects of contingent punishment with each of the aforementioned leader behaviours between weeks 15 and 26 of training. Results revealed that an interaction between contingent punishment and individual consideration (leader support) significantly predicted higher levels of mentally tough behaviour and individual recruit performance. In Study 2, the measure of contingent punishment was modified to reflect the recruits’ perceived threat of punishment, rather than punishment received. A longitudinal design, with data gathered at weeks three, eight and twelve, was used to examine the interacting effects of the threat of punishment and individual consideration on mental toughness and performance during the first 12 weeks of training. The results revealed a significant interaction at weeks 8 and 12 and a significant correlation between mentally tough behaviour and performance. Significant differences in mental toughness were also evidenced between recruits who withdrew from training and recruits who completed training. Chapter 4 describes a quasi-experimental study using a total of 173 Parachute Regiment recruits, divided into treatment and control conditions, to examine the efficacy of a three-week psychological skills intervention to develop mental toughness in elite military training. The intervention was delivered between weeks 16 and 20 of training, prior to the recruits attending a week-long physically and mentally demanding selection program. A contextually modified version of the Test of Performance Strategies-2 (TOPS-2) was used to measure the recruits’ use of psychological skills during training and during the selection program, while the MTMTI was used to measure recruits’ mentally tough behaviour at both pre and post intervention. Results revealed significantly greater use of goal-setting, relaxation techniques, self-talk strategies and imagery/mental rehearsal, and significantly higher levels of observer-rated mentally tough behaviour in the treatment group between pre and post intervention. However, during the selection program, significant differences were only evidenced with the use of relaxation and imagery. Individual recruit performance was shown to be significantly higher in the treatment group during the selection course. The final chapter discusses the findings of the thesis and provides suggestions for future research.