The Head Start CARES demonstration evaluated the effects of three distinct classroom-based approaches to enhancing children's social-emotional development on a large scale. The Incredible Years Teacher Training Program focuses on teachers' management of the classroom and of children's behavior. Preschool PATHS uses structured lessons to help children learn about emotions and interact with peers appropriately. Tools of the Mind--Play is a one-year version of the Tools curriculum that promotes children's learning through structured "make-believe" play. A comprehensive professional development package (including teacher training, ongoing coaching, and related technical assistance) supported delivery of the enhancements over the course of one year. The demonstration was conducted with 17 Head Start grantees that generally represent the diversity of Head Start settings nationally. Head Start CARES rigorously evaluated the impacts of the interventions, or "enhancements," by randomly assigning approximately 100 Head Start centers within the grantees to a program group that received one of the interventions or to a control condition without any of them. The estimated impacts should be interpreted as the effects of the enhancements beyond any effects of the existing Head Start program in these classrooms. Head Start CARES tested each enhancement's impacts on teachers' practices and on children's outcomes in the spring of the preschool year, comparing those impacts with the team's theory of change for each approach. (author abstract)