In the UK alone, roughly 700 in every 100,000 individuals will sustain an acquired brain injury (ABI) annually, including non-progressive damage to the brain such as traumatic brain injury, stroke, anoxia, or pressure from a brain tumour. Roughly half of those with ABI will experience depression. Therapies for depression have variable efficacy in ABI, possibly due to high demands on cognitive capacities that may be impaired post-injury. Behavioural approaches to treating depression that place lesser demands on cognition, such as Behavioural Activation (BA) therapy, offer promise. BA theory posits that depression reduces energy levels and expectations of positive outcomes from activities, and increases avoidance, which exacerbates depression. This dissertation provides a test of BA theory by investigating novel predictors of mood and activity levels, and outlines two interventions aimed at increasing activity levels and improving mood, in adults with an ABI from a wide variety of recruitment sources. I utilise both quantitative (e.g., structural equation modelling and mixed-effects models) and qualitative (e.g., interpretive description, thematic analysis) methodology to harmonise the individual perspective of ABI and to identify common patterns in ABI samples. The first empirical chapter concerns how mood disorders, activation and avoidance relate to intolerance of uncertainty (IU), a set of negative beliefs about uncertainty and its implications, in ABI. I demonstrate that IU can be reliably assessed in ABI, and that despite changes in uncertainty due to COVID-19, IU remains a robust predictor of poor mental health, lower activity levels, and greater avoidance in ABI. When controlling for ABI symptoms, the inhibiting effects of uncertainty provided unique variance in anxiety disorders but not depression. Although not as strongly associated with decreases in activity level, higher IU was associated with increased avoidance and rumination. The second empirical chapter focuses on discrepancies in estimation of functional capabilities (“impaired self-awareness”), specifically whether an individual with ABI overestimates or underestimates their ability relative to a family member. Using assessment data from over 200 adults assessed in NHS ABI neurorehabilitation services, those who overestimate themselves tended to be less depressed and anxious compared to those who underestimate and accurately estimate themselves, and had more severe injuries. However, those who overestimate themselves did not fare worse in long-term participation outcomes. The third empirical chapter uses qualitative methodology to explore perspectives of individuals with an ABI on changes in activity levels, and their views on various BA components used to increase activity levels. Level of perceived effort, its potential emotional impact and relation to participant values, and whether the component was used at the “right time” post-ABI were central themes when attempting to increase activity level. An investigation of two 8-week group interventions, designed to increase activity levels and mood in ABI, is presented in the final empirical chapter. Changes linked with each group are compared to waitlist controls in a pilot feasibility randomised controlled trial. The first intervention, “traditional” BA, uses therapeutic strategies and directly encourages individuals to plan and engage in reinforcing activities beyond group sessions. The second intervention, “experiential” BA, encourages participants to engage in reinforcing activities during group sessions only. Both interventions were feasible and acceptable, with approximately 50% - 60% of participants demonstrating clinically meaningful improvements in activity levels. In exploratory analyses of efficacy, both interventions improved mood, but only “traditional” BA resulted in lower anxiety, reductions in the impact of the inhibiting effects of uncertainty, and improvements in motivation. Analysis of qualitative data indicated that “traditional” BA seems to challenge negative self-beliefs and enhance confidence through planning activities, while “experiential” BA provides a low-pressure environment to engage in reinforcing activities. This dissertation demonstrates that increasing activity levels to improve mood in adults with ABI is an easily scalable and potentially effective therapeutic target, in alignment with BA theory. These data point towards potential expansions of BA theory in ABI, in particular the role of uncertainty and impaired self-awareness to activity level and their contributions to mood. This thesis concludes by stating that in clinical research, a mixed-methods approach to the application of relevant theory and intervention design is essential.