This thesis investigated the longitudinal role of defeat and entrapment in populations where these factors were expected to be particularly relevant (a sample of individuals from areas of socioeconomic deprivation and a sample of formal caregivers). The thesis then considered whether defeat and entrapment influenced reward sensitivity on a gambling task and lastly, designed a short scale measuring defeat and entrapment suitable for use in clinical populations. The research incorporated a review of the literature, two longitudinal studies, a behavioural study and the development of a scale. The literature review presented in Chapter 1 provided evidence of a well-established link between defeat, entrapment and poor mental health, suggesting that defeat and entrapment may act as a transdiagnostic process; contributing to the development and maintenance of a range of mental disorders. However, the review also demonstrated that defeat and entrapment relate to the same experiences, suggesting that logically these constructs may equally co-occur, although the structure of the constructs is currently debated. The studies presented in Chapters 3 and 4 demonstrated that perceptions of defeat and entrapment predicted poor mental health (depression and anxiety, and depression and caregiver burden) at a second time point, 12 months later. These chapters presented the first longitudinal evidence for samples recruited from the general population and occupational settings. Furthermore, these chapters provided evidence that the relationship between defeat, entrapment and poor mental health operates in a bidirectional way within a sample recruited from community settings, but a linear way within a sample of formal caregivers, suggesting that further research is needed to confirm the direction of this relationship. The research presented in Chapter 5 found a non-significant relationship between defeat and entrapment and reward sensitivity among a sample of undergraduate students. This may have arisen due to the generally low levels of defeat and entrapment within the sample despite highly varied performance on the task. Replication of this research within a sample where a wider range of defeat and entrapment experiences would be expected might be beneficial. Additionally, this thesis aimed to confirm the factor structure of defeat and entrapment amongst various populations. Exploratory and confirmatory factor analysis demonstrated that defeat and entrapment are best conceptualised as a single psychological construct (Chapters 3 and 6), supporting one-factor theories of defeat and entrapment (e.g. Taylor et al., 2011a). During the course of conducting research for this thesis, it became apparent that the length of existing scales used to measure defeat and entrapment were not suitable for use with clinical populations. Despite evidence that defeat and entrapment may reduce symptoms of mental health problems, their measurement has not yet translated to clinical practice. Although several reasons underlie this, a lack of short measurement tool is a major factor. To address this, an eight-item scale was developed, which demonstrated good psychometric properties across four samples from clinical and non-clinical settings. The current research was supported by a discussion of the clinical implications of the work, specifically identifying how defeat and entrapment could be implemented within therapeutic interventions for mental health problems. The current thesis represents a significant contribution to original research considering defeat and entrapment as predictors of mental health problems. The thesis presents the first longitudinal evidence that defeat and entrapment impact on mental health problems for individuals recruited from community and occupational settings and first application of defeat and entrapment to a behavioural task. Through development of a short scale, the thesis also presents a potential avenue to increase the measurement of defeat and entrapment in clinical settings.