Content and Language Integrated Learning (CLIL) is a dual-focused educational approach whereby an additional language is used for the learning and teaching of both content and language. In the Netherlands, this takes place in bilingual secondary education (tweetalig onderwijs). Policy guidelines, teaching handbooks, research and teacher education primarily focused on how subject teachers implement CLIL. Little was known about the nature and range of the pedagogical and collaborative practices of language teachers in this context. Exploring formal and practical theories of teaching, this dissertation reports on four studies; a literature review, focus group study, survey, and multiple-case study. These generated building blocks for a knowledge base for Teachers of English in Bilingual streams (TEBs) including a theoretical framework for language teaching in CLIL contexts, a set of practices which emerged as a professional development tool for TEBs, eight case descriptions of prototypical practices, and a model of the dynamic interaction of TEBs’ beliefs and practices. Reviewing the findings in the light of developments in conceptualizing what CLIL means for teachers in practice, the discussion highlights four points. Firstly, language teaching in CLIL contexts is not the same as foreign language teaching. Secondly, CLIL achieves integration through subject-specific language. Thirdly, CLIL contexts can lead to transformative change in language teachers’ beliefs and practices. Fourthly, collaboration between language and subject teachers can be beneficial. It concludes that teacher education and policy guidelines can and should do more to support, encourage and enable language teachers to be both creators and agents of change.