When it comes to teacher preparation, the support provided to new teachers, and ongoing professional development for working teachers, Grades K-12 and the field of early care and education (ECE) are two very different worlds. While both worlds assume that teachers' classroom skills and behavior can be influenced at multiple points in time (through pre-service education, during the first years of teaching, and over the course of a teacher's career) they differ along numerous dimensions. This paper describes some of those differences, discusses how those differences drive divergent research agendas in the two fields, and concludes with some recommendations for improving understanding across different levels of education in an effort to forge more integrated and effective policy, research and practice. Two general recommendations are included for building a coordinated research agenda: (1) Researchers, policymakers, and practitioners are encouraged to abandon the view of K-12 as one world, and ECE as another, and to approach all of their efforts with an eye to recognizing and understanding differences, working toward shared terminology, and building collaborative research agendas that will enable both arenas to learn from one another; and (2) Development of a national ECE workforce data system to provide information that is compatible with state and nationally collected data about K-12 teachers. A discussion of Value-Added Modeling is appended. (Contains 5 footnotes.) [For "Preparing Teachers of Young Children: The Current State of Knowledge, and a Blueprint for the Future. Executive Summary", see ED505293. 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.]