This thesis aimed to explore the pivotal role that psychological processes play in Acquired Brain Injury (ABI) sequelae at a both conceptual and interventional level. Initially, a systematic review and meta-analysis examined the available evidence base for the efficacy of psychological interventions in reducing aggressive behaviour following an ABI. In line with the PRISMA guidelines, a literature search identified eleven studies that met the inclusion criteria. As many studies within the neurorehabilitation literature use single-case methodology, the current meta-analysis adopted a novel approach enabling the synthesis of empirical data from both group design and single-case experimental design studies. The results of the meta-analysis demonstrated moderate effect sizes across both types of research design, suggesting significant reductions in aggressive behavior following psychological intervention. Maintenance effects were also reported, but should be interpreted with caution. A second cross-sectional study explored the discrepancy between subjectively reported cognitive impairment (SCI) and objectively measured cognitive impairment (OCI) following ABI, whilst highlighting the potential role of psychological factors. Twenty-four participants completed objective neuropsychological assessments and a series of psychometric questionnaires assessing psychological affect and perceived cognitive difficulties. A correlation analysis revealed no significant association between objective and subjective cognitive impairment. Conversely, psychological affect, such as anxiety and low mood, demonstrated a significant positive relationship with subjective cognitive impairment. An additional hierarchical regression analysis revealed psychological affect as a significant predictor of subjective cognitive impairment. The regression model found objective cognitive impairment to be non-significant. These findings suggest that an individual’s subjective experience of their cognitive difficulties may not be associated with their actual objective cognitive impairment. Other psychological factors may play a more crucial role in patients’ appraisals of their cognitive impairments. The limitations and clinical implications for both papers are discussed.