Background/Context: The current research on teacher knowledge and teacher accountability falls short on information about what teacher knowledge base could guide preparation and accountability of the mainstream teachers for meeting the academic needs of ELLs. Most recently, research on specialized knowledge for teaching has offered ways to understand the tasks of teaching that constitute the work of teaching a subject and a set of content-based problems. However, in this paper, we have argued that this domain does not address whether or not teaching academic content to English language learners (ELLs) involves any specialized knowledge for teaching. We sought to understand what specialized knowledge base for teaching, if any, is included in the work of teaching content to special student populations such as ELLs. In this exploration, we drew on the main perspective from literature about Systemic Functional Linguistics and academic language. Purpose: Grounding the theoretical argument based on these two areas of research, we propose the teachers' use of Disciplinary Linguistic Knowledge (DLK) for academic discourse of a discipline or content area. DLK is proposed as the knowledge base needed to facilitate ELLs' understanding of oral and written discourse within a discipline and their accurate use of language to engage them in the disciplinary discourse. Findings/Results: DLK refers to teachers' knowledge of a particular disciplinary discourse and involves knowledge for (a) identifying linguistic features of the disciplinary discourse and (b) modeling for ELLs how to communicate meaning in the discipline and engaging them in using the language of the discipline orally or in writing. We offer examples illustrating how teachers' knowledge of Disciplinary Linguistic Knowledge might manifest itself when teachers engage in the work of teaching content to ELLs. Conclusions/Recommendations: The use of DLK as a specialized knowledge base for teaching content to ELLs might help to further specify the role of teachers' knowledge of students within the larger research area of content knowledge for teaching. Also, operationalizing DLK as an assessment construct could address the need for next generation teacher assessments that aim to measure teachers' knowledge base for teaching content to ELLs.