The research evaluated the client impact and active ingredients of Dialectical Behaviour Therapy (DBT) for Borderline Personality Disorder (BPD) in three Adult Mental Health settings in Dublin. A mixed methodology approach was undertaken with two phases incorporating a single case study design and qualitative investigation. Phase 1 was a qualitative investigation of DBT clients who had completed (n=16) and those who dropped out (n=6) of treatment. The interview schedule examined client perspectives of treatment components, changes occurring since before DBT and reasons for dropping out. Phase 2 was a case study approach with clients who were currently attending DBT (n=10). Outcome measurements were administered and qualitative investigation was undertaken at two-month intervals to gauge perspectives of treatment and changes occurring. Records of attendance and admissions were also used. Four conclusions from the research were drawn. 1) DBT was associated with tangible improvements in client lives and had considerable impact on level of risk, well-being and functioning. Level of hopelessness and BPD symptoms showed significant positive change between baseline and six months. 2) DBT was not suitable for all participants for specific reasons discussed within. The level of satisfaction with DBT was overall high and some useful clinical recommendations were gleaned from service user input. 3) Key active ingredients of and obstacles to treatments were evaluated. 4) Recommendations for service delivery, development and further research were outlined.