Introducing a general overview and description of language teachers within the framework of TESOL, the chapter shifts focus to the process of becoming a bilingual language teacher and to the characteristics of language teachers in bilingual education. As such, the focus of the chapter is to develop a definition of ‘bilingual teachers’, and their role in bilingual education. Discussing theoretical assumptions and the changing role of language teachers, the chapter is centred around the idea of the growing worldwide need for language teachers to engage in bilingual instruction. Based on heteroglossic approaches to bilingual instruction, discussed in Chaps. 2 and 3 , the chapter highlights—in view of this extremely rapid growth, and the commitment to improving emergent bilinguals’ academic achievement—the pressing need to (re)define what it means to be a language teacher. It seeks an ideological clarity, particularly within settings previously described as monolingual or EFL contexts, considering the need for language teachers to develop their bilingual teaching competence.