Research has often focused on how to improve quality in early care and education (ECE) classrooms, but the question of how to ensure effective policy-level supports for such quality improvement remains unanswered. This paper introduces a theory of change that describes pathways from policy intervention, through high quality ECE, to young children's learning and development. It focuses on how policy intervention can change the culture of ECE programs and classrooms in order to best support both adults' and children's learning. This model is motivated by empirical evidence and grounded in bioecological theory, the sociocultural approach to learning, and the framework for social setting action. It argues that policies that encourage the creation of supportive professional environments for adult learning may be a critical ingredient for improving classroom quality and children's cognitive, social-emotional, and behavioral learning. Implications for studying policy interventions that target ECE quality are discussed. (author abstract)