This chapter outlines the relationship between Bahasa Indonesia and Malay (Bahasa Melayu), the official languages of the modern states of Indonesia and Malaysia. We trace the historical development of these two varieties of Malay from their classical source, Malay, before the advent of government by the colonial powers, and focus our discussion on factors resulting in the divergence that is visible today. We also discuss efforts to standardize the two postindependence languages, including the language policies adopted by the major Malay-speaking countries in the region. The chapter analyzes the differences between Bahasa Indonesia and Malay in terms of vocabulary and grammatical structure and advances certain hypotheses for some of the major remaining differences. Given the geopolitical proximity of Brunei and Singapore we cannot but include what happens to Malay there.