This handbook offers a comprehensive exploration of psychotherapy and behavior change, synthesizing cutting-edge research on treatment effectiveness across diverse therapeutic modalities. It delves into critical factors influencing outcomes, including patient characteristics, therapist skills, and the quality of the therapeutic alliance. The text evaluates the efficacy of psychodynamic, cognitive-behavioral, humanistic-experiential, and systemic therapies for various psychological conditions across different age groups. Additionally, it examines the mechanisms of change within therapy, identifying key techniques that drive improvement. The book also addresses therapist training, supervision, and the impact of qualitative research on advancing clinical practice. With a focus on evidence-based insights, this resource provides valuable implications for practitioners, researchers, and policymakers in the evolving landscape of mental health care.