Rates of suicidal thoughts and behaviours are heightened amongst prisoners, and present a significant challenge to correctional facilities globally. Despite this, there is a paucity of theoretically driven research examining the factors that underlie suicidality in prisoners. Two theoretical models of suicide, the Cry of Pain model (CoP; Williams, 1997) and the Schematic Appraisals Model of Suicide (SAMS; Johnson, Gooding & Tarrier, 2008) have highlighted the roles of negative appraisals and perceptions of defeat, entrapment, and hopelessness, as key psychological drivers for suicidal thoughts and behaviours. The overarching aim of this thesis was to investigate the psychological mechanisms that underlie suicidal thoughts and behaviours amongst prisoners. A corollary aim was to examine the psychological factors that may confer resilience to suicidal thoughts and behaviours amongst incarcerated individuals. Firstly, a comprehensive narrative review examined evidence of the applicability of current theoretical approaches to suicide, as applied to prisoner samples. The findings of this review highlighted gaps in the literature, from which a number of research questions were developed for investigation in the current thesis. Next, three empirical studies were designed to investigate the roles of perceptions of defeat, entrapment, hopelessness, and negative appraisals in suicidal ideation. In the first of these studies, cross-sectional evidence was obtained that perceptions of internal entrapment and hopelessness were predictive of suicidal ideation amongst prisoners (Chapter 4). In a second study, the predictive effects of defeat, hopelessness, and entrapment were examined in a longitudinal investigation, finding no significant longitudinal relationship (Chapter 7). In a further empirical study, support was provided for the role of momentary negative appraisals of the present and future as proximal predictors of the severity of suicidal thoughts (Chapter 5). Two further studies examined the role of impulsiveness in suicidality, and provided evidence for the deleterious effect of impulsiveness upon both suicidal ideation (Chapter 8) and self-harm ideation (Chapter 9) in prisoners. Two further studies provided the first theoretically driven investigations of potential resilience factors, conceptualized as positive self-appraisals, within a prisoner sample. Based on the Schematic Appraisals Model of Suicide (SAMS), it was proposed that positive self-appraisals would confer resilience against suicidal thoughts and behaviours. Two studies investigated this hypothesis. The first of these studies found that, contrary to predictions, positive self-appraisals of social support and social reciprocity did not buffer the impact of negative situational appraisals upon suicidal thoughts (Chapter 5). In the second study, evidence was obtained for a buffering effect of positive self-appraisals upon suicidal thoughts. In particular, positive appraisals of interpersonal problem-solving were found to buffer the effects of internal entrapment on suicidal thoughts (Chapter 6).Overall, the findings of this thesis serve to further our understanding of the psychological processes underlying the development of, and resilience to, suicidality amongst prisoners. These results underscore the need to empirically examine the applicability and transferability of psychological models of suicide within prisoner populations. Theoretical and clinical implications of these findings are outlined throughout the thesis.