The demand for English in Thailand has been increasing rapidly due to socio-political developments and the continuing growth of the economy. For Thai students, being proficient in English can enhance their education and career opportunities. To develop students’ English proficiency, both state and private schools are allowed to operate different types of bilingual education, including content and language integrated learning (CLIL). However, despite CLIL being implemented in Thailand for a decade, teachers are still grappling with many challenges because of limited understandings of CLIL and a lack of support from stakeholders. One of the challenges is related to the specialist language required for CLIL, which differs from that used in daily communication or general English classes. Teachers’ knowledge of the language and pedagogical approaches that may be used for CLIL could be developed through appropriate training and support, but there is a lack of CLIL research in the Thai context illuminating how teachers manage CLIL language and help students to learn effectively. The present study aimed to gain an insight into pre-service EFL teachers’ language-driven CLIL practices and beliefs about effective CLIL implementation. It adopted a multiple interpretive case study to investigate (1) how pre-service EFL teachers explain subject-specific terms and concepts to students; (2) negotiation of meaning (NoM) strategies that they adopt to address the difficulties in the explanation of subject-specific terms and concepts; and 3) their beliefs associated with effective CLIL implementation. Under these aims, two main research questions and one sub-research question were addressed: 1. How do Thai pre-service EFL teachers explain and negotiate the meaning of subject-specific terms and concepts in CLIL lessons? 1.1 What are the NoM strategies that the pre-service EFL teachers adopted to address difficulties concerning the explanation and students’ comprehension of the subject-specific terms and concepts? 2. What are Thai pre-service EFL teachers’ beliefs regarding effective CLIL implementation? The participants consisted of six pre-service EFL teachers teaching at two Thai secondary schools as part of a one-year teaching practicum for their Bachelor of Education programme. Data were gathered from June 2020 to December 2020 through semi-structured interviews, CLIL classroom video observations, and stimulated recall interviews. The data were transcribed and then analysed following the grounded theory approach by Charmaz (2006) and Hadley (2017). The study reveals five main NoM strategies that the teacher participants used in their CLIL lessons to explain subject-specific terms and concepts: students’ schema activation, students’ comprehension check, language and content modification, the use of visual supports, and the use of L1. When encountering teaching difficulties, the majority of the teacher participants used L1 to address the difficulties. However, despite their willingness to use L1 to negotiate meaning in the observed classes, the teacher participants expressed mixed beliefs regarding how and when L1 should be used. Furthermore, the findings demonstrated three key factors that the teacher participants believed to be influential in effective CLIL instruction: adequate CLIL teacher education, support from schools and stakeholders, and teaching preparation. The teachers highlighted that subject-specific language and language for learning and instructional purposes were two essential areas of CLIL teachers’ language proficiency. Finally, the findings showed that the teacher participants played several roles in language-driven CLIL implementation, including being language teachers, content teachers, and learners. Context specificity and the shortage of CLIL materials required the teacher participants to be analysts of students’ needs as well as materials evaluators and developers. The teachers’ educational backgrounds, their experiences as teachers and learners, and their school contexts were important factors in shaping the teacher participants’ professional identities and their roles as CLIL teachers. This research has shed light on pre-service EFL teachers’ NoM strategies in language-driven CLIL and their beliefs about effective CLIL implementation. It has contributed to the contextual, theoretical, and practical understandings of teachers’ beliefs and language-driven CLIL practices. The findings of this research also offer practical implications for language-driven CLIL practice, professional development, and teacher training to prepare EFL teachers for language-driven CLIL both in Thailand and other similar contexts.