This chapter reviews the timing and nature of Southeast Asia's earliest urbanism, to trace changes in form from the first millennium CE to the second millennium CE, and the Classical period from the ninth to fourteenth centuries CE. It addressees methodological concerns and contextualizing urban trajectories in the first and second millennia CE by focusing on mainland Southeast Asia for the first millennium CE, where such settlement has been documented. The chapter examines some ways in which Southeast Asia's early urban tradition was intimately tied to ritual practice and political performance. Early Southeast Asian cities were exemplary centers that shared key structural features, reflecting pan-regional systems that populations materialized through construction and ritual practice. The contemporary moated and walled site of Angkor Borei, southern Cambodia, encircles a 300-hectare area, and was linked to a series of settlements into the southern Mekong Delta to the site of Oc Eo.