Taken together, the contributions in this special issue give an excellent account of the themes currently on the agenda of language and language education researchers working on CLIL/immersion at the school level. If we take a 'historical' perspective on the types of research that have been pursued in the area, we notice that typical of the early phases of research were studies focussing on learning outcomes and the overall effectiveness of the approach(es). In the case of CLIL this was almost exclusively directed at language learning outcomes while the longer history of research on immersion in North America has also produced studies on content learning outcomes. Another early strand of research has looked at students and teachers in terms of their motivations, attitudes, and needs, while a third substantial strand of research has sought to characterise CLIL/immersion classrooms as environments of language use/learning, employing a wide array of concepts that are discourse analytic in a broad sense and subscribe to a broadly usage-based and contextual theory of language learning. A few years ago, the content learning perspective started to make an appearance in this research-strand via the notion of subject literacy (e.g. Llinares, Morton, and Whittaker 2012). The article by Hüttner and Smit (this issue) is a good example of this kind of theoretical framing and research approach: it is an observational discourse study, but with the focus on learners performing acts of argumentation as instantiations of subject literacy and thus content learning. The content subject studied is itself a new aspect in school-level CLIL/immersion research, namely Business Studies and Economics, where the ability of arguing a factual position appropriately is seen as a key subject literacy element. The observational mode is also employed by Morton (this issue) for the purpose of substantiating his conceptualisation of the different types of language knowledge which CLIL/immersion teachers need. Morton's contribution is also a good example of the productive migration of theoretical constructs between related research areas, as he develops his specific conceptualisation of language-knowledge for content-teachers from earlier constructs emerging in mathematics education and EFL. Applying a metaphor introduced by Dale, Oostdam, and Verspoor (2017), then, the 'lineage' of Morton's LK-CT (Language Knowledge for Content Teachers) reaches back to Shulman's notion of Pedagogical Content Knowledge or PCK (Shulman 1987).The lineage-metaphor is used by Dale et al. in their article to capture the 'several lines of thinking about language teaching in school contexts, broadly represented by the three terms foreign language (FL) teaching, second language (SL) teaching and mother tongue or first language (L1) teaching.' They continue by stating that 'All three lineages can provide insights into language teaching in this context. CLIL, CBI and immersion are built on both FL and SL lineages; EAL and Sheltered Instruction on an SL lineage; language and ...