South Khorasan Province ( Ostān-e Khorāsān-e Jonūbī) is one of the 31 provinces of Iran, located in the eastern part of the country. Birjand is the centre and capital of the province. At the time of the National Census of 2006, the province had a population of 600,568 in 159,255 households. The following census in 2011 counted 662,534 people in 183,114 households. In 2014, it was placed in Region 5. At the most recent census in 2016, the population of South Khorasan province was 768,898 in 223,984 households. This new province, is but the old Quhistan which was included into greater Khorasan in the Iranian administrative planning. However, historically Qohistan forms a separate entity, with a distinct culture, history, environment and ecology. South Khorasan is one of the three provinces that were created after the division of Khorasan in 2004. While at the beginning, the newly created "South Khorasan" included only Birjand County and some new counties detached from that county (i.e. Nehbandan, Darmian and Sarbisheh), in subsequent years, all northern and western cities and territories of the old Quhistan (such as Qaen, Ferdows and Tabas) have been annexed into South Khorasan, which as of 2016 consists of 11 counties. History Greater Khorasan has witnessed the rise and fall of many dynasties and governments in its territory throughout history. Various tribes of the Arabs, Turks, Kurds, Mongols, Turkemen and Afghans brought changes to the region time and time again. Ancient geographers of Iran divided Iran ("Iran-Shahr") into eight segments of which the most flourishing and largest was the territory of Greater Khorasan. Esfarayen, among other cities of the province, was one of the focal points for the residence of the Aryan tribes after entering Iran. The Parthian empire was based near Merv in Khorasan for many years. At Parthians times, Esfarayen was one of the important villages of Nishapur. During the Sassanid dynasty the province was governed by a Spahbod (Lieutenant General) called "Padgoosban" and four margraves, each commander of one of the four parts of the province. Khorasan was divided into four parts during the Muslim conquest of Persia, each section being named after the four largest cities, Nishapur, Merv, Herat, and Balkh. In the year 651, the army of Islamic Arabs invaded Khorasan. The territory remained in the hands of the Abbasid clan until 820, followed by the rule of the Iranian Taherid clan in the year 896