My intention for this thesis was to explore the factors that contribute to mental toughness development by considering the perspectives of individuals during critical stages of personal growth, and to embed such understandings within established theories from broader fields of psychological enquiry. Initially, I explored the innate and environmental factors that adolescents believed contributed to mental toughness, as well as attempted to ground such knowledge within an established theory from broader fields of psychological enquiry. I recruited eighteen adolescents (9 boys, 9 girls, Mage = 15.6 years) with a 'reputation' of mental toughness and invited them to participate in focus group interviews. Seven of these adolescents also participated in follow-up 1-1 interviews. Both focus groups and 1-1 interviews began with questions pertaining to the conceptual components of mental toughness (e.g., "what allows you to regularly perform to the best of your abilities?") and its development (e.g., "where did these characteristics originate from?"). Inductive analyses of participants' perspectives revealed that mental toughness development was predicated by four factors: interactions with significant others, the provision of supportive social processes, exposure to critical incidents, and a personal propensity for curiosity. These findings were interpreted within the context of Bronfenbrenner's (2001) bioecological model. I concluded that, while my findings were one of the first to advocate for the combined role of innate and environmental factors in the development of mental toughness, it is the latter that is of greatest interest from an applied perspective and should be more closely considered in the future.Subsequently, I sought to focus my efforts on the environmental factors that contribute to mental toughness development. Although the bioecological model is a useful framework for categorising the factors associated with mental toughness development, it is not easily applied to practice. That is, the framework is more descriptive than practical in nature. As such, I contested that there was a need to draw on another theory that, although consistent with the bioecological model, possesses strong applied implications.iii As such, I investigated the utility of Self-determination theory (SDT, Deci & Ryan, 1985b) principles for understanding mental toughness development. I predicted that coaching environments (i.e., autonomy-supportive, controlling) would be related to mental toughness indirectly through psychological needs satisfaction, and that psychological needs satisfaction would indirectly relate with performance and psychological health (i.e., positive/negative affect) through mental toughness.I recruited 221 adolescent cross-country runners (136 male and 85 female, Mage = 14.36) and invited them to complete questionnaires pertaining to their perceived coaching environments, the degree to which their psychological needs were satisfied, mental toughness, and psychological health. Performance was measur...