Around 500 BC, the introduction of iron working technology caused radical changes in social structure in both mainland and insular Southeast Asia. The most important phenomenon, which had strong impacts in all aspects of life, was the transformation from small and egalitarian to large and complex societies. This complexity laid the foundations for the emergence and development of early states in the area. The causes and dynamics of these processes were various in nature and included both internal and external elements. Using site and artefact distributions in the Thu Bon Valley, one of the largest and most fertile riverine areas in the whole of central Vietnam, we examine the nature and evolution of regional and interregional interaction between Sa Huynh communities and the outside world, especially with the Han Chinese, the Dong Son of northern Vietnam, and contemporary cultures in South and Southeast Asia.