The present introduction to the Special Section on Mechanisms of Change in Treatments of Personality Disorders explains the value of research focusing on processes of change in psychotherapy by referring to a variety of methodologies. Whereas outcome for these treatments has been repeatedly demonstrated, it remains an open question what the core psychobiological features of change are. Taking a radically empirical perspective, this introduction focuses on patient and therapeutic relationship variables, such as emotional and socio-cognitive processing. The six empirical articles of the Special Section are introduced, as are the three discussions of the Special Section.