Excerpts from the executive summary: No ECE program can succeed without teachers who can establish warm and caring relationships with children, light the fires of children’s curiosity and love of learning, and foster their development and readiness for school. But what is the best way to prepare skilled and effective teachers of young children? And how can ECE programs best support teachers in continuing to learn and grow as professionals, implementing the approaches to early care and education that they have been taught? These questions have major implications for policy, practice and research in the early care and education field, where, for many years, the entry requirements to work as a teacher have been very low. Although teachers in many publicly funded preschool and Head Start programs are now required to obtain a bachelor’s degree and a specialization or certification in early childhood education, expectations for staff in other ECE programs typically remain limited to a certain number of training hours or college credits, well short of a degree. Given an increasing emphasis on evidence-based policy and practice, many have turned to the existing research literature—both from ECE and from K-12 education—for answers about the most appropriate and effective types of teacher preparation and professional development. Ideally, the scientific wisdom and evidence accrued in one sector of education should inform and advance research, policy, and practice in the other. But because infrastructures and career pathways are so different in these two fields, researchers in K-12 and in ECE have tended to pose questions and formulate answers in dramatically different ways. The purpose of this two-part paper is to help bridge the worlds of ECE and K-12, and to help shape a coordinated research agenda, by examining their differing vantage points, language, and terminology, and the current state of knowledge about the effective preparation of excellent teachers. Part I summarizes the differences between these two fields, but finds more than enough similarities to warrant a close consideration, in Part II, of the combined wisdom of both fields, and of what remains to be learned, about teacher preparation—concluding with a set of key recommendations for research and policy.