Book excerpt: This book presents a validated observation instrument to support and further develop the pedagogic expertise of teachers of English Learners in US schools. Based on sociocultural and second language teaching and learning theories, the Observation Protocol for Academic Literacies (OPAL) has a variety of uses across teacher preparation programs, pre-K-12 classroom contexts and school districts. This book leads the reader through the process of using OPAL with real classroom observations in the form of vignettes, in order to develop confidence and reliability scoring evidence in research-based practices. The book provides examples of its non-evaluative uses to conduct research, support teachers and identify areas of strength and weakness in their professional development. It also offers researchers and policymakers a tool for collecting classroom-based evidence to inform the implementation and refinement of English Learner programs.