Shia Islam (Arabic: شيعة‎, Shīʿah) is the second largest denomination of Islam. Adherents of Shia Islam are called Shi'as, Shi'ites, or Shias. "Shia" is the short form of the historic phrase Shīʻatu ʻAlī (شيعة علي), meaning "followers of Ali", "faction of Ali", or "party of Ali".

Like other schools of thought in Islam, Shia Islam is based on the teachings of the Quran and the message of the Islamic prophet Muhammad. In contrast to other schools of thought, the Shia believe that only God has the right to choose a representative to safeguard Islam, the Quran and sharia (based upon verses in the Quran which stipulate this according to the Shia). For this reason, the Shias look to Ali, whom they consider divinely appointed, as the rightful successor to Muhammad, and the first imam. The Shia believe that there are numerous narrations where Muhammad selected Ali as his successor.

Shias believe that Muhammad's family, the Ahl al-Bayt ("the People of the House"), and certain individuals among his descendants, who are known as Imams, have special spiritual and political authority over the community. Therefore, Shias prefer hadith attributed to the Ahl al-Bayt and close associates, and have their own separate collection of hadiths. All Shias agree on the succession of Hassan and Hussein after Ali, but they may differ after Hussein. Hassan and Hussein are described by Shias as "leaders of all youths in Paradise", and believe that these sons of Ali were the true leaders and caliphs of the Muslims. Shias regard Ali as the successor of Muhammad not only ruling over the community in justice, but also interpreting Islamic practices and its esoteric meaning. Hence he was regarded as being free from error and sin (infallible), and appointed by God by divine decree (nass) to be the first Imam. Ali is known as "perfect man" (al-insan al-kamil) similar to Muhammad, according to Shia viewpoint.