This paper argues that teachers' mental lives represent the ‘hidden side’ of teaching. It examines how teacher learning and teacher knowledge, as central attributes of those mental lives, have been conceptualized and studied since 1975 and traces connections to similar work in English language teaching (ELT)In this paper, I use the phrase ‘English language teaching,’ or (ELT), to refer to the teaching of English as a second, additional, or foreign language, known in the US as TESOL. I recognize that the phrase ELT is a bit problematic in that in the United States it can be taken to refer to English as mother tongue or first language instruction (language arts), while in Europe it would seem to be more accurately applied in this case.. While the majority of literature reviewed is drawn from the north American perspective, parallels are sketched in some of the emerging research in ELT teacher education. The analysis examines four broad families of issues: how teachers learn content and teaching practices, how teachers' mental processes are conceived, the role of prior knowledge in learning to teach, and the role of social and institutional context. Taken together, research in these areas suggests implications for the design and practice of teacher training and professional development in Second language teacher education.