Low‐income preschool children face many risks to their social emotional development that can affect their school experience and social outcomes for years to come. Although there are some promising approaches to improving young children’s social‐emotional competence, the evidence base is limited, particularly on the effectiveness of these approaches when operating at large scale. This report describes the Head Start CARES (“Classroom-based Approaches and Resources for Emotion and Social skill promotion”) demonstration, which was designed to test strategies for supporting children’s social-emotional competence in Head Start classrooms. The demonstration included large-scale implementation of three social-emotional approaches in a variety of Head Start classrooms and provided the same professional development model, technical assistance, and program monitoring to support each of the three enhancements, in order to help ensure that they were implemented as designed. Authors focus on how the three enhancements and the related supports were delivered, by examining various measures of fidelity. The demonstration was conducted by MDRC, in collaboration with MEF Associates and several academic partners.