This is a study of Korean contributions to cultural changes in ancient Japan as it developed agriculture and increasing social complexity and finally formed the Yamato state over the course of a thousand years, between 400 B.C. and A.D. 600. Central to this study are three broad themes, supported primarily by archaeology but importantly informed by historical texts. First, key cultural features and technologies that were essential to increasing social complexity in Yayoi period Japan and to formation of a centralized state in the sixth century A.D. entered the archipelago directly from the Korea Peninsula. Second, a dominant factor behind the infusion of Korean cultural features was the movement, in several waves, of peninsula residents into the Japanese archipelago. While trade moved peninsula goods to the archipelago all throughout the formative period, Korean technologies, skills, ideologies, and cultural systems moved with people, including permanent immigrants, temporary residents, and official envoys. The Korean immigrants in particular were impelled initially by explosive population growth in Korea fueled by the spread of agriculture there and later by increasingly tumultuous political and military events that unfolded in the peninsula as rival polities contended for power during several hundred years of war. Third, a number of Korean immigrants emerged as powerful technocrats and political functionaries during the Kofun period, providing important organizational experience and service to the Yamato court during the process of state formation in Japan.