Dialectical behavior therapy (DBT) can be considered a well-established treatment for borderline personality disorder (BPD) as evidenced by seven well-controlled randomized clinical trials across four independent research teams. The primary purpose of this article is to address a variety of potential mechanisms of change that may be associated with those aspects of DBT that are unique to the treatment and its theoretical underpinnings. Based on the biosocial theory of BPD, many of these mechanisms can be distilled down to the following process: the reduction of ineffective action tendencies linked with dysregulated emotions. Specifically we address the following interventions and associated mechanisms of change: mindfulness, validation, targeting and chain analysis, and dialectics. Patient change in BPD is conceptualized primarily as helping the patient to engage in functional, life-enhancing behavior, even when intense emotions are present. Ultimately, our goal was to provide guidance for theoretically and empirically grounded research on the mechanisms of change in DBT. © 2006 Wiley Periodicals, Inc. J Clin Psychol 62: 459–480, 2006.