Approaches to standards, curriculum development, and teaching are diverse. However, increasingly learning trajectories are being used as a basis for each of these. In this chapter, we present our own definition and use of the construct, positing that learning trajectories can serve as one effective foundation for scientifically-validated mathematics standards, curricula, and pedagogy. We discuss the current state of learning trajectories in early childhood mathematics and present the development and expansion of the concept in our work, which began with studies of individuals and progressed to large scale-up implementations and evaluations. Throughout this work, learning trajectories served as the structure that maintained coherence.