This article analyzes relations among the peoples of the Korean peninsula and the Japanese archipelago between A.D. 350 and 700 from an archaeological perspective. Until recently, the international academic community was divided between those who advocated the "horserider theory" and those who argued that the Japanese court (located in the Kinai) controlled the three southern Korean states of Kaya, Paekche, and Shilla between 350 and 562. This work seeks a middle position consistent with all the evidence. Scholars from Japan, South Korea, China, Western Europe, and the United States have shown that the peninsular peoples transmitted a huge volume of materials, technologies, ideas, and institutions to the archipelago. These items included: iron and iron-working techniques, iron weapons and armor, horse trappings, new agricultural tools and practices, stoneware, the household oven, gold and silver metallurgy, stone-fitting methods, silk-weaving, writing, plans for mountain fortifications, the crossbow, Buddhism and its architecture, and methods of statecraft such as the be, kabane, law codes, and ranking. Peoples of the Korean peninsula acted as middlemen, often altering or refining ideas that originated elsewhere. Four mechanisms encouraged the influx: minimal trade, large-scale immigration, some plundering by Japanese troops fighting in Korea, and the foreign policies of states such as Paekche and Koguryŏ. Japanese chieftains anxious to receive technology transfers from the Korean states were obliged to send troops to intervene in the peninsular wars on behalf of the donor government. While this work does not support either extreme interpretation, it underlines the Japanese debt to the peninsular peoples, and points out that the rise of complex states in Japan and Korea was intimately interrelated.