The theoretical, clinical, and empirical foundations of psychotherapy come from five primary movements that still exist today, continue to evolve, and remain scientifically productive: psychodynamic, cognitive-behavioral, humanistic, systemic, and integrative. The goal of this chapter is to examine the philosophical, clinical, and scientific underpinnings of each of these major traditions in detail. Experts in these five approaches will describe (a) the model of psychopathology (especially focusing upon etiological and maintenance factors emphasized in assessment and case formulation); (b) the focus and specific techniques used in treatment planning and implementation; (c) the hypothesized therapeutic mechanisms of change; and (d) the outcome literature/empirical support for each modality. We conclude with a look toward the future of the science of psychotherapy and the scientist-practitioner model of psychotherapy.