This chapter traces the historical development of CLIL from early bilingual education initiatives and situates it within European and global language policies. It describes the 4Cs model—Content, Communication, Cognition and Culture—which underpins CLIL’s dual-focus philosophy. The model emphasizes that subject matter and language learning should progress together, using three language categories: the language of learning (disciplinary concepts), language for learning (structures necessary to engage in classroom tasks) and language through learning (emergent language that arises from interaction). The chapter also compares CLIL across subject areas, explaining that while it thrives in sciences and humanities, its implementation in PE is inconsistent. It identifies challenges such as uneven teacher training, variable program intensity and a lack of standardized frameworks for balancing content and language instruction. By clarifying foundational concepts, this chapter sets the stage for the more specialized discussion of CLIL in PE.