This article reports on a randomized controlled trial of the First Step to Success early intervention that was conducted over a 4-year period in Albuquerque Public Schools. First Step is a selected intervention for students in Grades 1 through 3 with externalizing behavior problems, and it addresses secondary prevention goals and objectives. It consists of three modular components (screening, school intervention, parent training); lasts approximately 3 months; and is initially set up, delivered, and coordinated by a behavioral coach (e.g., school counselor, behavior specialist, social worker). Project Year 1 of this efficacy trial was devoted to gearing-up activities (e.g., hiring, training, planning, logistical arrangements); Years 2 and 3 each involved implementing First Step with approximately 100 behaviorally at-risk students. Students, teachers, and classrooms were randomly assigned to either intervention or usual care comparison conditions. Year 4 activities focused on conducting long-term, follow-up assessments and implementing sustainability procedures to preserve achieved gains. Pre-post teacher and parent ratings of student behavior and social skills showed moderately robust effect sizes, ranging from 0.54 to 0.87, that favored the intervention group. Direct measures of academic performance (oral reading fluency, letter-word identification) were not sensitive to the intervention. The implications and limitations of the study are discussed. (Contains 3 tables and 1 figure.)