Research Findings: Preschool behavior problems are of increasing concern to early childhood educators. Preventive interventions are being developed, but implementation in underresourced child care programs is challenging. This study describes the implementation of an adapted Second Step curriculum to increase children's social skills and decrease behavior problems in preschool classrooms. Teacher training/coaching, organizational supports, and procedures for sustainability are described. Measures included baseline classroom characteristics, lesson completion and fidelity, and teacher satisfaction. Teachers completed 88% of the lessons across 2 years. Follow-up monitoring in Year 3, once all training was terminated, found that 95% of lessons were delivered with higher fidelity than in the first 2 years. Teachers were satisfied with the training, felt that the curriculum improved children's social skills, and stated that they would continue to use the curriculum. Lesson completion and fidelity were associated with various teacher characteristics, training attendance, classroom characteristics, teacher and child turnover, and the percentage of children with developmental delays in the classroom. Practice or Policy: The results support the implementation strategy and suggest that paying attention to teacher morale, general skill development, and a period of ongoing support for training is important and will lead to sustainable and high implementation rates. (Contains 5 tables.)