The thesis contains five chapters that attempt to extend our understanding of Mental Toughness (MT) in relation to personality, swimming performance, training behaviours and MT behaviours. The thesis focuses predominantly on swimming environments, which can be very stressful and often require athletes to train long hours and perform under intense pressure. As such, it provides a useful context to investigate MT. Chapter 1 critically reviews some of the previous MT research in the domains of qualitative research and quantitative research. The Chapter introduces personality as a possible trait explanation of MT and proposes that, even though training behaviours has been indicated as an important source of MT, it is yet relatively unexplored. The Chapter then sets out a series of questions upon which the thesis is based. Chapter 2 focused on three aims. First, an informant rating of MT in swimming was developed (Coach Rated MT). Second, to replicate previous findings (Hardy, Bell, & Beattie, 2014), revised Reinforcement Sensitivity Theory (McNaughton & Gray, 2000) was used to predict coach rated MT behaviour. Our findings supported that of Hardy et al. (2014), that is, when reward sensitivity is low, increasing levels of punishment sensitivity positively related to MT behaviour; but when reward sensitivity was high, increasing levels of punishment sensitivity negatively relate to MT behaviour. Third, the thesis set out to investigate whether punishment and reward sensitivities could actually predict swimming performance. Our findings showed there was a significant punishment sensitivity and reward sensitivity interaction. That is, when reward sensitivity was low, as punishment sensitivity increased, swimming times improved. However, with high reward sensitivity as punishment sensitivity increased, swimming times reduced. However, findings showed that there was no significant correlation between coach assessed MT and actual swimming performance. 6 Chapter 3 examined the mediating role of training behaviours on self-report MT and MT behaviour in swimming. In this chapter swimmers completed three self-report MT measurements; the Psychological Performance Inventory-Alternative (PPI-A; Golby, Sheard, & van Wersch, 2007); the Sport Mental Toughness Questionnaire (SMTQ; Sheard, Golby, Wersch, 2009); and the Mental Toughness Index (MTI; Gucciardi, Hanton, Gordon, Mallett, &Temby, 2014). Swimmers and coaches also completed a measure of Self-Regulated Training Behaviours (SRTB) and the coach completed the Swimming MT Inventory (SMTI) to assess MT behaviour. Findings supported our hypothesis that self-regulated training behaviours (coach and self-rated) had a positive relationship with coach and self-rated MT. Further, training behaviours mediated the relationship between self-report MT and coach rated MT behaviour. Chapter 4, examined three main personality profiles of psychoticism, extraversion, and neuroticism in relation to MT and training behaviours. To assess the swimmer’s personality profiles we utilized the Eysenck Personality Questionnaire–Revised Short version (EPQR-S; Eysenck, Eysenck, & Barrett, 1985). Coaches completed the MT behaviour measure (CRTB) and the Quality of Training Inventory consisting of distractibility and coping with adversity. Findings revealed that swimmers characterized by high levels of both psychoticism and MT skills displayed higher levels of training and MT behaviours. That is, self-rated MT only had a positive relationship with training and MT behaviour when psychoticism was high. Chapter 5 concludes the thesis. More specifically, the chapter provides a summary and integrated discussion of the thesis findings, implications, limitations, strengths and avenues for future research.