This thesis evaluates the effectiveness of Schema Therapy (ST) for treating personality disorders by reviewing its theoretical background and existing research. Unlike traditional CBT which focuses on current symptoms, ST targets deep-rooted schemas and modes developed from unmet childhood needs. By analyzing various studies over the past twenty years, this work shows that ST significantly reduces core symptoms and improves patients' daily functioning. These positive outcomes are most proven in borderline personality disorder but also extend to other groups, such as Cluster C disorders. Although there are some methodological limitations in current research, the evidence suggests that ST is a highly valuable treatment that leads to long-term recovery.