Head Start, which is the largest federally funded early childhood education program in the United States, aims to increase school readiness among low-income children from birth to age five years by boosting their cognitive, social, and emotional development. The Head Start CARES ("Classroom-based Approaches and Resources for Emotion and Social skill promotion") demonstration was designed to expand the current evidence base by evaluating enhancements to the standard curricula that have been used in Head Start classrooms. The demonstration included (1) selection of three different strategies, or program "enhancements," that in smaller-scale tests showed positive effects on children's social-emotional outcomes, such as reducing problem behaviors and promoting positive peer relationships; (2) implementation of these three enhancements in many different kinds of classrooms that operate within the regular Head Start system; and (3) 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 while efforts were made to rapidly increase their scale, as Head Start CARES envisioned. This report, which focuses on how well the three enhancements and the related supports were implemented, is part of a larger Head Start CARES randomized control trial that is also examining the impact of the approaches on classrooms and the children in them. The Head Start CARES demonstration was conceived and sponsored by the Office of Head Start and the Office of Planning, Research and Evaluation in the Administration for Children and Families in the U.S. Department of Health and Human Services. The demonstration was conducted by MDRC, a nonprofit, nonpartisan education and social policy research organization, in collaboration with MEF Associates and several academic partners. (author abstract)