This paper offers a chain of evidence collected to identify and verify the domain of teacher knowledge and skills required to teach ELLs content effectively in mathematics, science, reading/English language arts, and social studies. We initially assessed this domain through a review of literature and state standards for nonspecialized teacher certification. We developed a set of 67 evidence statements in 2 categories: pedagogical knowledge and linguistic knowledge. We conducted a national survey of practitioners and teacher educators to validate these statements, receiving 269 responses. A panel of 14 teacher educators and teachers further validated the statements by reaching consensus that the statements support the claims of the assessment under development. We found that the domain of knowledge necessary to teach ELLs in the content areas is insufficiently defined in the standards and literature. However, we identified that across the content areas, teachers should have knowledge and understanding of the register of the particular discipline. Beyond merely understanding the linguistic demands of a given discipline, teachers should also be able to raise ELLs’ metalinguistic awareness of effective oral and written communication in the language of the content area. Also, teachers’ use of multimodal instructional materials was deemed commonly essential in assessing teacher knowledge. Finally, we acknowledge that the chain of evidence collected in this line of research is a small step taken forward to understanding what should be assessed and how. With that, we offer future directions for research.