Publisher Summary Psychologists have always been very interested in the effectiveness of the treatment they provide. The outcomes of treatment for many types of psychotherapy were normally assessed through conversation during the termination stage of therapy. Patients and their therapists commonly reflected on the progress that was made along with challenges that could be anticipated and invitations to return to therapy as needed. More recently, adding standardized outcomes assessment to these conversations has been found to be highly valuable as well. These are becoming more important priorities as demands for accountability grow and strategies are developed that can improve the quality of health care through the ongoing monitoring of responses to treatment. The potential for improving effectiveness through the monitoring of outcomes deserves special attention from clinicians and managers. Though outcome assessment was easily avoided in clinical practice in the past, it is becoming increasingly unavoidable. This is particularly the case when behavioral health care is viewed from the biopsychosocial perspective—the effectiveness with which patients' needs are met is a top priority from this perspective.