Borderline personality disorder (BPD) is a highly prevalent and chronic psychiatric problem characterized by a pattern of chaotic, self-defeating interpersonal relationships, emotional instability, poor impulse control, chronic anger, frequent suicidality, self-mutilation and has a high risk of suicide. Not surprisingly, BPD has been associated with higher levels of service use in emergency room, daycare, and inpatient settings. Although, preliminary evidence suggest that psychotherapy treatment is helpful in reducing borderline pathology, controlled studies are rare and the mechanisms that promote change remain unspecified. Clinical investigators have noted that fundamental aspects of borderline personality disorder can be understood as stemming from impairments in patients underlying attachment organization. The attachment theory constructs of internal working models, coherence, and reflective function provide an important means for understanding how psychotherapy may directly influence BPD. In a sample of 140 patients with BPD, randomly assigned to either a modified psychodynamic treatment called transference-focused psychotherapy (TFP), a cognitive behavioral therapy called Dialectic Behavioral Therapy (DBT), or a treatment as ususal. Supportive Psychotherapy (SPT), the current study using a multi-method approach to data collection, including self-report, interview, and global and micro- analytic coding of observational data, assesses the efficacy of these treatments and the mediating role of attachment, through examining effect sizes and growth curve analyses and regression analyses. By examining the psychological mechanisms that contribute to and mediate BPD this study hopes to inform psychological interventions and promote adaptation and optimal mental health.