This chapter presents the main aspects of a study conducted to explore existing beliefs and perspectives on CLIL-teaching specific practices and methodological aspects. The study aimed at identifying the educational practices accepted by CLIL experienced teachers and comparing them with prospective primary education teachers' underlying assumptions both before and after explicit instruction on CLIL based on experts' advice. The analysis of the results seems to indicate that instruction has modified students' beliefs and that specific training may contribute to the alignment of in-service teachers' praxis with the suggestions by experts in the field, in areas such as identifying problems in advance, using the learners' mother tongue, correcting language errors, using authentic materials, or presenting content. Interestingly, the answers of both teachers and students acknowledge the effectiveness of CLIL, although they reveal that appropriate qualification and training are a matter of concern that could benefit teachers greatly.