This chapter deals explicitly with Monsoon Asia as a concept. It calls for a revival of that concept in a form which has ecology and prehistory at its centre, and which takes its cue in the first place from French thinkers of the early twentieth century. While it pays due respect to Indologists like George Coedès and Sylvain Lévi, its main inspiration comes from the geographer Jules Sion and especially the sociologist Paul Mus, who argued that Indic civilization was underlain in South and Southeast Asia by a common cultural and religious substrate that both shaped its development and facilitated its diffusion. Elements of the prehistoric “religion of the monsoon zone”, in this view, included the cult of local ancestors and the idea of an autochthonous god of the soil, both of which were fused with new beliefs of “Aryan” origin as Indian and Indianized cultures developed. With its characteristic attachment to territory, water, and fertility, the common primordial culture was rooted in ecology and the intensive, sedentary agricultural systems of the monsoon countries.