Borderline personality disorder (BPD) is a complex condition whose severity is compounded by the heterogeneous psychological functioning of those who suffer from the disorder. These differences have made the identification of a unified treatment difficult. Attempts have been made to reduce the effect of this variation within the disorder by investigating subtypes of BPD. However, the clinical utility of this approach has not been examined. This book investigates the presence of subgroups of BPD and examines whether treatment effectiveness varies as a function of subtype. Using variables that are central to cognitive behavioural models of BPD, subgroups are identified and defined on the basis of whether participants attribute the causes of negative events as being themselves or other people. The findings of this research suggest that not only can theoretically valid subgroups of BPD be identified, but that these subgroups have clinical utility in understanding sufferers' responses to intervention. These analyses should be useful to both researchers and professionals seeking to better understand the complex nature of the borderline personality diagnosis.