This chapter draws on insights from the intersection between sociolinguistics and second language acquisition to review and discuss the diglossic nature of Arabic and its impact on the field of Arabic as a second language. After highlighting key concepts related to the focus of the chapter such as communicative competence, sociolinguistic competence, pragmatic competence, and standard language ideologies, the chapter discusses historical research and perspectives regarding diglossia as a major topic in Arabic sociolinguistics and underscores its impact on the field. Finally, the chapter concludes with recommendations for future directions in research and practices related to Arabic as second language.