The ultimate goal of this research is to facilitate rapid enhancement of youth substance abuse treatments by determining the key mechanisms of change (i.e., what to target more intensely to generate better outcomes). This work is critical because current outpatient adolescent substance abuse treatments yield only small to medium effects that diminish once treatment has ended. The proposed modern mediation study will address this serious public health problem by experimentally evaluating the most critical factors causing change in client outcomes during adolescent substance abuse treatment, directing the field to focus treatment efforts on those factors.