This report draws from decades of experience using observation in early childhood education, which has implications for administrative decisions, evaluation practices, and policymaking in K-12. Early childhood education has long embraced the value of observing classrooms and teacher-child interactions. In early childhood education the features of the settings in which children are served are the hallmarks of quality. These features can include health and safety considerations, the materials and physical layout of the space, and the interactions that take place between adults and children--such as conversations, emotional tone, or physical proximity. Standardized observations of these early childhood education features in turn yield metrics that are used in state and federal policy, program improvement investments, and the credentialing of professionals--all uses that K-12 education is now considering. This paper examines lessons learned from observation in early childhood education that may be helpful as states and districts begin implementing more rigorous observation protocols for K-12 teachers. Although these lessons apply to all grades, they may be particularly relevant for K-3 as assessment of student performance using standardized achievement tests is most challenging in those grades. These lessons focus on the importance of standardization, trained observers, methods for ensuring the validity and reliability of the instruments, and the use of observational measures as a lever to produce effective teaching. These lessons form the basis for the recommendations offered in this paper. (Contains 1 figure and 33 endnotes.)