Parents, professionals, and policymakers agree that quality is crucial for early education. But precise, consistent, and valid definitions of quality have been elusive. In this article, Robert Pianta, Jason Downer, and Bridget Hamre tackle the questions of how to define quality, how to measure it, and how to ensure that more children experience it. Definitions of quality in early education, the authors write, generally include four aspects. The first is a program's structural elements, such as length of the school day or teachers' qualifications. The second encompasses general features of the classroom environment, ranging from playground equipment to activities involving staff, children, or parents. Third are the dimensions of teacher-student interactions that children experience directly. Finally, aggregate indices--such as "quality rating and improvement systems"--combine measurements across types of program elements. Pianta, Downer, and Hamre find very little evidence that programs' structural features influence children's development. Instead, they zero in on teacher-student interactions-- characterized by teachers' sensitivity to individual needs, support for positive behavior, and stimulation of language and cognitive development--as a key indicator of classroom quality that appears to benefit all children from prekindergarten through third grade. Teachers' interactions with children can be significantly and systematically improved through targeted and sustained professional development. Yet efforts to improve the quality of such interactions at scale and to ensure that quality remains consistent from prekindergarten through third grade have so far been ineffectual. If we accept the evidence that direct experiences within classrooms are the best indicators of program quality, the authors argue, then the next wave of science and policy must refine and advance the definition, measurement, production, and consistency of these experiences in early education.