Personality disorders (PDs) are common, chronic, mental health problems. The majority of treatment outcome research, which has focused specifically on Borderline PD, has provided substantial empirical support for Dialectical Behaviour Therapy (DBT; Linehan, 1993), particularly in terms of self-harm reduction. Nevertheless, DBT graduates can continue to experience poor personality functioning across PD diagnostic categories, Axis I disorders, and restricted lives. Acceptance and Commitment Therapy (ACT; Hayes, Strosahl, & Wilson, 1999), might be suitable as a follow-up intervention for DBT graduates, to address their continued difficulties: to date, however, there has been little empirical investigation of its utility in relation to PD. This thesis was therefore designed to examine theoretical underpinnings of ACT relevant to the development of an ACT intervention for DBT graduates. Study 1 tested the performance of a new self-report measure of cognitive fusion (CF), the Cognitive Fusion Questionnaire (CFQ), with a mental health sample, including individuals with PD. CF is a key ACT concept, and the CFQ proved to be a psychometrically sound measure of CF with people with mental health problems. Study 2 used cross-sectional modelling to show that CF fully mediated the relationships between two PD risk factors, negative affectivity and childhood trauma, and personality functioning in adulthood. Study 3 used the CFQ to investigate the behavioural correlates of CF. These findings strengthened the possibility that an ACT-based intervention might prove effective in improving outcomes for DBT graduates. To explore this further, Studies 4 and 5 were designed as very small-scale uncontrolled treatment development trials for this population. Study 4 suggested that ACT had a positive impact on engagement in life, but produced little improvement in psychiatric symptomology. Study 5 tested a revised protocol, which yielded more consistently positive findings, with improvements in both engagement in life and psychiatric symptoms. These findings tentatively suggest that ACT may have a role to play as a DBT follow-up intervention.