The two East Asian millets, broomcorn (Panicum miliaceum) and foxtail millet (Setaria italica), spread across Eurasia and became important crops by the second millennium BC. The earliest indisputable archaeobotanical remains of broomcorn millet outside of East Asia identified thus far date to the end of the third millennium BC in eastern Kazakhstan. By the end of the second millennium BC, broomcorn millet cultivation had spread to the rest of Central Eurasia and to Eastern Europe. Both millets are well suited to an arid ecology where the dominant portion of the annual precipitation falls during the warm summer months. Indeed, the earliest sites with millet remains outside of East Asia are restricted to a narrow foothill ecocline between 800 and 2000 m a.s.l., where summer precipitation is relatively high (about 125 mm or more, from May through October). Ethnohistorically, millets, as fastgrowing, warm-season crops, were commonly cultivated as a way to reduce agricultural risk and were grown as a low-investment rain-fed summer crop. In Eurasian regions with moist winters and very low summer precipitation, the prevailing agricultural regime had long depended on winter wheat (Triticum aestivum) and barley (Hordeum vulgare) cultivated with supplemental irrigation. We propose that the secondary wave of millet cultivation that spread into the summer-dry regions of southern Central Asia is associated with an intensification of productive economies in general, and specifically with the expansion of centrally organized irrigation works.