This Element explains how cross-border mobility defines diplomatic relationships between Southeast Asian states and social and political dynamics within the region's key destination countries. It begins by providing an historically situated discussion of bordering processes within the region, examining evolving historical conceptions of power and sovereignty, and processes of bordering in colonial and post-colonial times. It then turns to the political, environmental, and economic drivers of contemporary cross-border mobility before examining governments' efforts to manage different kinds of border-crossers and the tensions that these efforts give rise to. Having discussed the politics of cross-border mobility in host communities, the Element returns to the question of why consideration of bordering practices and cross-border mobility is necessary in understanding contemporary Southeast Asia.