The second of the so-called Four Righteous Caliphs (al-khulafa al-rashidun). Born c. 586, died 644. Caliph 634-644, succeeded by Uthman ibn Affan.
Umar was an early convert to Islam, and close to Muhammad until the Prophet's death in 632. According to Sunni Muslim tradition, Muhammad's death was followed by a heated dispute among his followers over the issue of succession - a dispute which Umar defused by acclaiming Abu Bakr as legitimate successor and first Caliph. When Abu Bakr died, Umar was his clear successor.
In Umar's own reign, the Caliphate subjugated extensive territories in the Middle East, and Umar is supposed to have laid down the ground rules for caliphal administration.
Umar also dabbled in calendar reform, instituting the Hijra Calendar, still in use, which takes as its starting point the hijra or forced exile from Mekkah (Mecca) to Madinah (Medina) of the Prophet and his followers, in 622.
Umar was assassinated in Madinah in 644, but had previously set in place a commission to appoint his successor. The commission named Uthman ibn Affan.
Favorising his own family, the Umayyad, Uthman was soon charged with nepotism by his political opponents. Controversy also surrounded his pay cuts for soldiers, and his decision to order an authorised version of the Quran to be created, and all other versions to be burned.
A rebellion ensued, and Uthman was killed in Madinah (Medina) by units of the Caliphate army from Egypt and Iraq.