The military is a multifaceted organisation consisting of an extremely diverse workforce where multiple individual differences are found among personnel. Therefore, possessing knowledge of personality characteristics that are adaptive along with those that are maladaptive in training and combat situations would be very advantageous for the military, especially when recruiting and training new recruits. The present thesis contains five chapters with the intent to first explore individual differences that could account for recruits who pass or fail military basic training (MTB). A second purpose of the thesis was to develop a deeper understanding of the personality profiles of courageous actors who have been decorated for their acts of bravery. Chapter 1 offers brief empirical reviews on personalities and their relationship with performance in the military environment and identify certain gaps in the literature that needs to be addressed. In particular: (i) limited current empirical research is available on the effect that military basic training (MBT) may have on the personality of recruits under training; (ii) limited empirical evidence is available exploring the positive adaptation and functioning of psychoticism and psychopathy personality types within the military environment; (iii) limited research is available that explore the relationship between courageous soldiers and psychopathy; and (iv) limited empirical evidence is available on the positive effect of psychological attributes (i.e., punishment and reward sensitivity, mental toughness, effortful control, internalisation of core values) on performance of military recruits under training. The chapter concludes by proposing that the military creates an environment where individuals with some measure of psychoticism (Eysenck & Eysenck, 1976) and psychopathy (Lilienfeld & Widows, 2005) may flourish, provided that they are given appropriate training. The focus of Chapter 2 falls on psychological attributes and behaviours that British male Infantry recruits bring to the start of MBT and how those attributes influence retention and performance outcomes. Results indicated that at that start of MBT, high levels of psychoticism, mental toughness (MT), activation control and attention control, and low levels of neuroticism and punishment sensitivity successfully differentiated between recruits who successfully completed MBT first time from recruits who did not complete MBT first time. The results and theoretical implications are discussed in terms of what the Army may do to improve recruitment and retention. Chapter 3 is longitudinal in nature and explored the effect MBT has on the psychological attributes of military recruits during a 26 week MBT training programme. The same psychological attributes that were explored within Chapter 2 were investigated in Chapter 3. With relevance to personality, results indicated a significant increase in extraversion across training, whereas neuroticism and punishment sensitivity significantly decreased during MBT. With relation cognitive variables, only external regulation levels significantly decreased during MBT. The results and theoretical implications are discussed in terms of how training may (or may not) influence some deep-rooted characteristics of the recruit. Chapter 4 examines the ultimate expression of military performance; namely, the decorated courageous soldier. The chapter followed a multi-methodological approach involving qualitative and quantitative methods to explore the personality of courageous soldiers with specific reference, but not limited to: psychopathy, cognitive processes, motivation and behaviours. Findings indicated that decorated courageous soldiers are not psychopathic; however, decorated courageous soldiers share certain characteristics with psychopaths. The findings from Chapter 4 further suggest that courageous acts were performed as a result of the right person, with the right genetic-makeup, being in the right situation when it mattered. The results and theoretical implications are discussed in terms of individual differences in personality, behaviour, cognitions and motivation. Chapter 5 concludes the thesis by providing a summary of the empirical chapters, followed by a discussion of theoretical points of interest and applied implications. Some limitations and strengths of the thesis are highlighted; whereafter the thesis is concluded by forwarding future research questions in order to further understand personalities and their performance within the military environment.