Generalised anxiety disorder (GAD) is characterised by excessive and uncontrollable worry alongside physiological anxiety. It is the most common anxiety disorder and carries a chronic, relapsing course with high rates of non-response to treatment. The presence of aversive mental imagery has long been identified as a key feature of GAD that maintains worry, yet existing imagery-based interventions for GAD show limited results. Further, GAD is frequently considered a characterological condition that is underscored by maladaptive schema patterns. Imagery rescripting is a technique that alters maladaptive mental imagery and underlying schematic beliefs that has shown promise in treating a range of mental health conditions. Yet few studies have applied the technique to treatment of GAD and none have done so in a clinical setting. This thesis comprised four studies aimed to further characterise patterns of mental imagining in GAD, and to develop and trial an imagery rescripting intervention to treat worry and GAD symptoms. In Chapter 2, I outline the results of a two-arm systematic review investigating firstly patterns of mental imagery in this population and secondly the nature and effectiveness of existing imagery-based interventions to treat GAD. The review highlights a need for further development of interventions that target observed deficits in memory and future imagining. In Chapter 3, I explored the relationship between trait worry and properties of worry images, and demonstrated that a brief, self-guided imagery rescripting intervention targeting worry images in an online community sample could improve cognitive and affective responses to these images. Chapter 4 expanded on these procedures to investigate the effectiveness of a therapist-facilitated imagery rescripting intervention among undergraduate individuals high in trait worry. The intervention was equivalent to imaginal exposure in improving worry, GAD symptoms, and features of imagery. Chapter 5 presents a single case series trialling an 11-session protocol of imagery rescripting targeting worry images and associated childhood memories in 18 individuals with a primary diagnosis of GAD. The intervention was found feasible and acceptable, and produced improvements in primary outcomes of worry and GAD symptoms and secondary outcomes of depression symptoms and imagery features at post-treatment that were maintained at 3-month follow-up. This thesis provides preliminary evidence that imagery rescripting may be an acceptable and effective intervention for GAD. Results are interpreted in light of the strengths and limitations of these studies, and avenues for future research efforts seeking to apply imagery rescripting and other imagery-based methods in GAD are suggested. Theoretical considerations and implications for clinical practice are also discussed.