This digest examines the knowledge base for teaching, noting that many critical decisions about educational structure, policy, and assessment rely on it. The professionalization of teaching depends on showing that teaching requires mastery of a specialized body of knowledge that is applied ethically. Serious deliberation is needed in the professional community about types of knowledge required, conceptual frameworks for organizing and using knowledge, and modes of inquiry used in creating and validating knowledge claims. Several knowledge base frameworks have emerged recently, including two sponsored by national associations for teacher education. These frameworks have resulted in discussions about defining the knowledge base. Implications for teaching and teacher education include adding impetus to calls for change in educational curriculum and structure (thus changing preservice and beginning teacher programs). Five criteria for evaluating the extent to which conflicting conceptions of the knowledge base actually inform practice are: (1) relation of different types of knowledge to disciplined inquiry; (2) inclusion of multiple perspectives on teaching and schooling; (3) explanation of the interdependence of technical and normative aspects of teaching; (4) usefulness of the framework; and (5) degree to which it encourages reflective judgment and action. (SM)