Located on the Silk Road; Bukhara has for three millennia been a center of trade; scholarship; culture and religion. During the golden age of the Samanids; Bukhara became the intellectual center of the Islamic world. Part of the Persian Empire for several centuries; it is today mostly populated by Tajik-speakers. The historic center of Bukhara; which contains numerous mosques and madrassas; has been listed by UNESCO as a World Heritage Site.