Most of Tajikistan's population belongs to the Tajik ethnic group, who share culture and history with Afghanistan and speak the Persian language (officially referred to as Tajiki in Tajikistan). Once part of the Samanid Empire, Tajikistan became a constituent republic of the Soviet Union in the 20th century, known as the Tajik Soviet Socialist Republic (Tajik SSR).
Historically, Tajiks and Persians come from very similar stock, speaking variants of the same language and are related as part of the larger group of Iranian peoples. The Tajik language is the mother tongue of around 80% of the citizens of Tajikistan. The main urban centers in today's Tajikistan include Dushanbe (the capital), Khujand, Kulob, Panjakent and Istaravshan. Most of the population are members of the Sunni Islam faith, though there is also a sizable number of Ismailis.