The Common Core State Standards Initiative, in its aim to align diverse state curricula and improve educational outcomes, calls for K-12 teachers in the United States to engage all students in mathematical problem solving along with reading and writing complex text through the use of rigorous academic content. Until recently, most teacher evaluation systems in this country did not measure or promote the ability of teachers to practice in these ways. This report discusses efforts to develop and implement Common Core standards and assessments in the 45 states and the District of Columbia that are initiative members, and outlines how past attempts to enact standards-based reform have been impeded by limitations in teacher evaluation. It also draws on the notion of "standard of care," from the field of medicine, to note that advances in the understanding of subject matter, pedagogy, how students learn, and technology call for teachers to continually acquire new knowledge and to refine their instructional practices by participating in comprehensive professional development on a regular basis. Several new approaches to evaluating teachers hold promise for promoting the type of ongoing teacher learning and changes in instruction that would be associated with a professional standard of care in K-12 teaching. These approaches include classroom observation protocols, student surveys, value-added models, and teacher performance assessments. This report details these approaches and explains their potential to strongly support the enactment of the Common Core standards and assessments. At the same time, it also considers a number of challenges connected with implementing each of these. The approaches to teacher evaluation presented here, when combined with comprehensive professional development for teachers and school leaders and changes in the organization and capacities of school districts, have the potential to support the types of teacher knowledge acquisition and changes in instructional practices called for by the Common Core standards and assessments. (Contains 59 endnotes.)