Across the political spectrum, high-quality early care and education (ECE) is viewed as essential to educational reform. 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. Practitioners and policy makers are increasingly embracing higher qualifications for ECE teachers as one means for ensuring better care for children. In K-12 education, too, highly politicized issues are at stake, with ongoing controversy about the merits of certification, the value of college and university schools of education, teacher accountability for student outcomes, the best ways to measure teacher effectiveness, and how to guide new teachers through the critical first five years of service. Based on an increasing emphasis on evidence-based policy and practice, many have turned to the existing research literature for answers about the most appropriate and effective types of teacher preparation and professional development for both ECE and K-12 education. 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 significantly different in these two fields, researchers in K-12 and in ECE have tended to pose questions and formulate answers in fundamentally 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 the two fields, while finding enough similarities to warrant a close consideration, in Part II, of the combined knowledge of both fields, and of what remains to be learned, about teacher preparation, concluding with a set of key recommendations for research and policy. (Contains 2 footnotes and 1 table.) [For "Preparing Teachers of Young Children: The Current State of Knowledge, and a Blueprint for the Future. Part 1: Teacher Preparation and Professional Development in Grades K-12 and in Early Care and Education: Differences and Similarities, and Implications for Research", see ED505296. For "Preparing Teachers of Young Children: The Current State of Knowledge, and a Blueprint for the Future. Part 2: Effective Teacher Preparation in Early Care and Education: Toward a Comprehensive Research Agenda", see ED505298.]