To promote mental and physical well-being for all children, families and the early childhood workforce help promote social-emotional development in very young children every day. They are the experts on the social and cultural strengths of their communities and are positioned to provide the child- and family-centered care that is critical to child and family stability. Our measurement guide serves professionals working with very young children in defining social-emotional development for children within their teams, aligning practices to support that development, and harnessing measurement and evaluation tools and processes to promote those definitions and practices. (author abstract)