This chapter focuses on key issues relevant to the preparation and continuing professional development of teachers who teach academic content through English as a second or foreign language. This covers such approaches as immersion, English-medium instruction (EMI), and content and language integrated learning (CLIL). The umbrella term ‘Content-based language education’ (CBLE) is used to refer to all such approaches. The chapter reviews three key issues which affect all teacher education in contexts where the teaching, learning and use of the English language and academic content are combined. These issues are: the need for effective initial and in-service teacher education provision; the need for greater understanding of teachers’ training and professional development needs; the need for further research into teacher education programmes and their processes and outcomes. On a more theoretical level, the chapter also addresses the nature of teachers’ knowledge for this type of teaching: either of language or content knowledge taken separately, or of their integration. It also focuses on content and language integration as a question of identity, particularly the struggle for teachers to see themselves as responsible for both types of learning. In order to illustrate how some of these key issues play out in teachers’ reflections and classroom interaction, illustrative examples of data drawn from a research project on CBLE teachers’ knowledge and practices are presented and analysed. The chapter ends by identifying practical implications for teacher education and professional development and possible future prospects for research in this type of language education.