This article considers the importance of focusing on the patient's experiential world in the treatment process. It also presents an evidence-based conceptual model of personality development and psychopathology that enabled investigators to introduce aspects of patients into research designs so they could explore patient-treatment and patient-outcome interactions—interactions that address complex questions such as what kinds of treatments are most effective with what types of patients and that suggest that different, but equally desirable, therapeutic outcomes occur through differing mechanisms of therapeutic action. Consistent with clinical experiences discussed in the earlier sections of this article, results from three studies of long-term, psychodynamically oriented treatment, and from a randomized clinical trial investigating several brief manual-directed treatments for depression, demonstrate the importance of focusing on the contributions of patients to the treatment process in both clinical practice and research.