The second century B.C. was a period of upheaval in Central Asia: the powerful Hiung-Nu peoples were dispossessing weaker tribes of their pasture lands and forcing them to migrate westwards. Between 150 and 125 B.C. a succession of nomadic tribes, the last and most powerful of which were a branch of the Yueh-Chi, were driven down into Sogdiana. It is now generally held that these tribes were of Aryan origin, though the question is not perhaps settled with absolute certainty. Before long, however, a second group, the K’ang, possessed themselves of Sogdiana, driving the Yueh Chi on into Bactria and the Afghan mountains[1]. In these districts they found, alongside the Iranian peasantry, a settled population of Tukhari (in Chinese, Ta-Hia), already noted in the Chinese annals for their commercial enterprise[2], and while at first the nomad tribes introduced complete confusion, it would seem that they rapidly absorbed, or were absorbed by, the native elements, and thus assimilated the Hellenistic civilisation of Bactria. From this fusion arose, about 50 A.D., the powerful Kushan Empire which spread into India on the one side and probably maintained some form of suzerainty over the K’ang kingdoms of Sogdiana on the other. Under the new empire, Buddhism was acclimatised in Turkestan, and Sogdiana developed into a great _entrepôt_ for Chinese trade with the West. Towards the close of the third century the Kushan Empire, weakened by attacks from the new national dynasties in India and Persia, reverted to its primitive form of small independent principalities, which, however, retained sufficient cohesion to prevent a Persian reconquest. It is practically certain that Sāsānian authority never extended beyond Balkh and rarely as far. In the fourth and fifth centuries references are made to a fresh horde of nomads in the north-east, the Juan-Juan (Chionitae, Avars)[3], but it does not appear that any new settlements were made in the Oxus countries. In the middle of the fifth century, another people, the Ephthalites (Arabic Hay_t_al, Chinese Ye-Tha), perhaps a branch of the Hiung-nu, not only completely overran the former Kushan territories, but by successive defeats of the Persian armies forced the Sāsānid Kings to pay tribute. The Ephthalites appear to have been a nomadic people organised as a military caste of the familiar Turkish type, and the existing institutions and principalities, in large part at least, continued side by side with them[4]. Their rule was too transitory to produce any lasting effects, or to inflict any serious injury on the commerce and prosperity of Sogdiana.