Zaiddiyah (also: Zaidi, Zaydi, or in the West Fivers) refers to a sect within ShiaIslam. Zaidi separated from the main Shia branch (the "Twelvers") over a disagreement as to who the fifth Imam was. Twelvers believe it was Muhammad al-Baqir, while Zaidis hold that it was his half-brother, Zayd bin Ali.
Zaidi see Zayd as the fifth Imam because of the rebellion he led against the Umayyad dynasty, which he believed was corrupt. Muhammad al-Baqir did not engage in political action, whereas Zayd preached that a true Imam must fight against corrupt rulers.
Zaidi beliefs are moderate compared to other Shia sects. The Zaidis do not believe in the infallibility of the Imams, nor that they receive divine guidance. Zaidis also do not believe that the Imamate must pass from father to son, but believe it can be held by any descendant of Ali. They also reject the Twelver notion of a hidden Imam, and like the Ismailis believe in a living imam, or even imams.
In matters of law or fiqh, the Zaidis are actually closest to the Sunni Shafie school.
Zaidis form the dominant religious group in Yemen, and the leader of the Zaidi community took the title of Caliph. As such, the ruler of Yemen was known as the Caliph, and this system continued until the middle of the 20th century (1962).
Zaidis also do not believe that the Imamate must pass from father to son, but believe it can be held by any Sayyid descended from both Hasan ibn Ali or Husayn ibn Ali.
Zaidis believe Zayd was the rightful successor to the ImÄmate because he led a rebellion against the Umayyads, whom he believed were tyrannical and corrupt.
The first Zaidi state was established in Tabaristan (northern Iran) in 864 C.E.; it lasted until the death of its leader at the hand of the Samanids in 928 C.E. Forty years later the state was revived in Gilan (north-western Iran) and survived under Hasanid leaders until the 12th century.