The fourth millennium BC was a critical period of socio-economic and political transformation in the Iranian Plateau and its surrounding zones. This period witnessed the appearance of the world's earliest urban centres, hierarchical administrative structures, and writing systems. These developments are indicative of significant changes in socio-political structures that have been interpreted as evidence for the rise of early states and the development of inter-regional trade, embedded in longer-term processes that began in the later fifth millennium BC. Iran was an important player in western Asia especially in the medium- to long-range trade in raw materials and finished items throughout this period. The 20 papers presented here illustrate forcefully how the re-evaluation of old excavation results, combined with much new research, has dramatically expanded our knowledge and understanding of local developments on the Iranian Plateau and of long-range interactions during the critical period of the fourth millennium BC.