For years, the alternation between L1 and L2 was banned in language classes, since it was thought that this would impair the learning of the second language. In recent years, however, there has been a recognition of the validity of alternation. As such, this book investigates the alternation of the L1 and L2 in CLIL and EMI contexts at three educational levels (primary, secondary, and tertiary) in Italy. Using a mixed-methods research approach, it shows that alternation played a mainly lexical role to explain specific lexis or at least to provide the students with lexis in both languages (English and Italian). It highlights that lexis in CLIL and EMI contexts is indeed crucial for both content and language learning. The book also argues that, despite the contextual differences that exist, the three educational levels (primary, secondary, and tertiary) show a very similar use of alternation, thereby pointing at similarities more than differences.