This study addresses the role of code switching to students' L1 (Arabic) in their ESL classrooms and whether it expands interaction in these classrooms. The gap perceived in this area needs to be addressed towards the domains of sociolinguistics and applied linguistics in the ESL classrooms teaching environment. Henceforth, this study draws on data collected from basic, secondary and college ESL classrooms in the Sudan and Saudi Arabia. The study incorporates various data gathering procedures: audio-taped spoken data of some ESL classrooms, questionnaire and semi-structured interviews. The data has been analysed by descriptive statistics. The findings generally indicate that CS has been used extensively, purposefully and functionally as part and parcel of ESL classrooms' discourse. The overall findings suggest that, although the use of L1 has been criticized in the existing literature, yet it has been admitted by ESL teachers, showing that L1 use is unavoidable at basic, secondary and tertiary level in the Sudan and Saudi Arabia. In classrooms where both students and teachers share the same L1, there is a great tendency for using it in the fields of explaining meaning and difficult words, guiding interpretation, transmitting lesson content, illustrating grammatical rules, organizing ESL classrooms and praising and encouraging students. Thus, L1 has been found useful in expanding the interactions of ESL classrooms towards facilitating ESL learning process. The study also calls for sensitizing both teachers and students about the helpful uses of CS. Therefore, syllabi and methods of teaching ESL should incorporate CS in an occasional and judicious way.