Young children who enter school without sufficient social and emotional learning (SEL) skills may have a hard time learning. Yet early childhood educators say they do not get enough training to effectively help children develop such skills. In this article, Megan McClelland, Shauna Tominey, Sara Schmitt, and Robert Duncan examine the theory and science behind early childhood SEL interventions. Reviewing evaluation results, they find that several interventions are promising, though we need to know more about how and why their results vary for different groups of children. Three strategies appear to make interventions more successful, the authors write. First, many effective SEL interventions include training or professional development for early childhood teachers; some also emphasize building teachers' own SEL skills. Second, effective interventions embed direct instruction and practice of targeted skills into daily activities, giving children repeated opportunities to practice SEL skills in different contexts; it is best if these activities grow more complex over time. Third, effective interventions engage children's families, so that kids have a chance to work on their SEL skills both at school and at home. Family components may include teaching adults how to help children build SEL skills or teaching adults themselves how to practice and model such skills. Are early childhood SEL interventions cost-effective? The short answer is that it is too soon to be sure. We will not know how the costs and benefits stack up without further research that follows participants into later childhood and adulthood. In this context, we particularly need to understand how the long-term benefits of shorter, less intensive, and less costly programs compare to the benefits of more intensive and costlier ones.