English as an Additional Language (EAL) as a professional discipline is a significant component of public education, and a main educational response to the growing ethno-linguistic diversities in many English-speaking countries. In this article I will focus on the ways in which professional language teacher knowledge has changed over time, paying particular attention to the disciplinary content bases, and the likely influences of current research in language education and applied linguistics on our understanding of EAL pedagogy. The overall purpose is to provide a view on the likely 'assemblage' of professional knowledge and skills informing EAL practice in light of the continuously evolving professional sensibilities and disciplinary concerns, and its implications for the principal components underlying the development of teacher professionalism.