The term code-switching (CS) that occurs in everyday situations, or naturalistic code-switching, has been a controversial strategy regarding whether it benefits or impedes language learning. The aim of this study was to investigate CS in conversations between teachers and students of ESP classes in order to explore the types and functions of CS usually used in the teaching and learning process, and to investigate the teachers' and students' opinions about CS in classroom practice, particularly in English for nursing and tourism classes. The study found that code-switching was one of the major strategies in these ESP classes. CS in the teaching process could be categorized according to the following: inter-sentential switching, intra-sentential switching, extra-sentential switching, and code selection. The CS functions were both pedagogical (asking questions, giving explanations, translating, and emphasizing ideas) and social functions (directing, reminding, injecting humor, and encouraging students). The majority of the teachers demonstrated positive opinions about CS and agreed that CS from English to Thai could benefit their classes, as it is one of the most effective strategies for helping their students obtain clear ideas on the subject matter. This strategy not only made the students understand more of the lesson but also increased their confidence and comfort in the teaching process. The CS characteristics and opinions found in this study may provide a guideline for language teachers and policymakers, especially in EFL countries in the AEC and in this global era.