During the mid-first millennium AD period considered in this chapter, a variety of polities along the coasts of the western façade of Insular Southeast Asia, all of them with long-standing relationships with polities across the Bay of Bengal, started adapting to their own needs a South Asian cultural package comprising writing, the usage of Sanskrit, text-based religions, and associated literature, iconography, and architecture. Recent work on these polities has reassigned such developments to a manifold process, with a variety of incentives and outcomes, showing that local rulers and their people, far from being passive recipients of imported cultural traits, were not cut off from transformations and developments in world economy taking place elsewhere in Asia at the turn of the first millennium and played an active part in such sociopolitical transformations. This chapter first concentrates on the state of Srivijaya and on those polities of the Thai-Malay Peninsula that were associated with the latter, for which both archaeological and textual evidence is available to document the state-formation process. Only then does this chapter try to recognize comparable processes for other areas—Java, Bali, Borneo—for which archaeological data remains scattered.