Clements’ presentation focused primarily on the second of the questions. He presented a model of how he believes research on young children’s learning should proceed, without commenting directly on the ways in which research is currently influencing practice. He began by showing a set of slides of children of the same age demonstrating very different competencies, and asked: “What possible theory of curriculum in research is going to help us address [children at disparate levels] and help us figure out what best to do?” As he sees it, no theory, or even definition, of what a preschool curriculum should be is guiding current work or providing a framework for thinking and planning. What is needed is a true science of curriculum in mathematics, science, and other fields. By this he means a view of curriculum development that goes beyond the provision of practical feedback to those who develop curricula. He views the development of curricula as a form of inquiry that “provides reliable ways of dealing with experiences and achieving goals.” Clements presented examples of the kinds of questions about curriculum he thought such a science of curriculum could help to address, with particular attention to its relationship to practice, policy, and theory; see Table 3-1.

Clements and his colleagues have developed an operating framework for thinking about curriculum research. Such research can begin with an a priori foundation, a broad philosophy of learning rooted in past research that yields a starting notion of the way children learn. Such research can also be organized around learning models, or, as he termed them, learning trajectories. These trajectories are pathways that children typically take through a series of levels or