The Shia represent the second largest denomination of Islam and adherents of Shia Islam are called Shias or the Shi'a as a collective or Shi'i individually.

Shi'a is the short form of the historic phrase Shīʻatu ʻAlī meaning "followers", "faction" or "party" of Muhammad's son-in-law and cousin Ali, whom the Shia believe to be Muhammad's successor in the Caliphate.

Twelver Shia (Ithnā'ashariyyah) is the largest branch of Shia Islam and the term Shia Muslim is often taken to refer to Twelvers by default.

Shia Muslims constitute ten to twenty percent of the world's Muslim population and thirty-eight percent of the entire population of the Near and Middle East.

Shi'i Islam is based on the Quran and the message of the Islamic prophet Muhammad attested in hadith recorded by the Shia, and certain books deemed sacred to the Shia (Nahj al-Balagha).

The Shia, in contrast to other Muslims, believe that only God has the right to choose a representative to safeguard Islam, the Quran and sharia.

The Shia thus look to Ali, Muhammad's son-in-law, whom they revere and consider divinely appointed, as the rightful successor to Muhammad, and the first Imam.

The Shia In the centuries after the death of Muhammad extend this "Imami" doctrine to Muhammad's family, the Ahl al-Bayt ("the People of the House"), and certain individuals among his descendants, known as Imams, who they believe possess special spiritual and political authority over the community, infallibility, and other quasi-divine traits.

Modern Shi'i Islam has been divided into three main groupings: Twelvers, Ismailis and Zaidis, although there are myriad Shi'i subsects.