|Ja'far ibn Muhammad|
|Predecessor||Muhammad ibn Ali|
|Successor||Musa ibn Ja'far|
His contribution and influence, however, are far wider than the Shia Islam school of thought. He is cited in a wide range of historical sources, including al-Tabari, al-Ya’qubi, and al-Masudi. Sunni, Sufi’ and Shi’i sources all give testimony to his influence.
His father was Muhammad al-Baqir and his grandfather was Ali Zain al-Abideen. His great grandfather was Husain ibn Ali grandson of the Holy Prophet who was martyred at Karbala. His mother was Umm Farwa daughter of Qasim, the son of Muhammad ibn Abu Bakr. Qasim was a very pious person and a devout student of Ali Zain al-Abideen. He was regarded amongst the seven top most jurists of Medina during his times. 
Jaffar al-Sadiq married Fatima, daughter of Hassan ibn Ali. She bore him two sons, Ismail and Abdullah. After the loss of his first wife, he married Hamida Khatun, a slave of Berber origin. She also bore him two sons, Musa al-Kadhim and Muhammad al-Dibaj. Imam al-Sadiq also had three other sons and daughters; Isaac, Abbas, Ali, Fatima, Umm Farwa and Asma. 
Under Umayyad and Abbasid Rulers
Al-Sadiq lived in a very violent era which saw the downfall of the Umayyad Dynasty and the rise of the 'Abbasid caliphate. The last five Ummayad rulers (Hisham bin ‘Abdul Malik, Walid bin Yazid, Yazid bin Walid, Ibrâhim bin Walid, and Marwan al-Himar) were facing too much internal conflict and rebels to harass al-Sadiq, who had explicitly rejected the idea of arm rebellion.
A number of traditions are reported in the Shii hadith books about the designation of al-Jaffar Sadiq as the successor of his father. 
[Muhammad b. Abi Umayr reported on the authority of Hisham b. Salim on the authority of Abu Abd Allah, Jafar' b. Muhammad, peace be on them, who said:]' When my father was near to death he said: "Jafar I give testamentary enjoinment to you (to treat) my followers well." "May I be your ransom," I replied, "by God, I will make them (know their religion so well) that any man among them in the country will not (have to) ask anyone (for advice)."
[Aban b. 'Uthman reported on the authority of Abu-al-Sabbah al- Kinam, who said:] Abu Jafar Muhammad, peace be on him, looked towards his son, Abu Abd Allah Ja'far, peace be on him, and said (to us): "Do you see that man? He is one of those of whom God, the Mighty and High, said: We wish to grant a favour to those who have been humiliated in the land and we will make them Imams and inheritors [XXVIII 5].
Jaffar al-Sadiq was the Imam or the spiritual leader for the Shia for thirty-four years.
Jaffar al-Sadiq was regarded as Imam and taught hadith and legal fiqh to the Shia and others who became prominent later as authorities among the Sunnis. A number of the most active theological thinkers of the time were associated with him and with his son, Musa al-Kadhim.  Al-Sadiq trained thousands of disciples in diverse fields such as theology, jurisprudence, and Arabic grammar. Some of the famous Shia theologians and jurists like Hisham b. al-Hakam, Zurara b. Ayyan, and Muhammad b. Muslim were his students. 
He is credited with establishing the most important criterion for testing hadith, whatever that is in agreement with the Quran should be accepted while anything that is contrary to the Quran must be rejected. This came to be regarded as the most important principle in judging traditions.
The governor of Medina by the order of the Abbasid caliph Al-Mansur poisoned Imam al-Sadiq. He was 65 years old. The funeral prayer was conducted by his son Musa al-Kadhim. His body was laid to rest in the Baqi cemetery in Medina. 
The great majority of the Shias began to follow the Imamate of his son Musa al-Kadhim upon his death. The following sons of Imam al-Sadiq were regarded as his successors, briefly.
The Fathites sect began to follow the leadership of Abd Allah al-Aftah, the eldest son of Jaffar al-Sadiq. This sect disappeared with the death of al-Aftah since he did not leave any descendents. 
The Fatimid Ismailis accepted the Imamate of Ismail ibn Jaffar al-Sadiq. Ismail was not al-Sadiq successor since he is believed to have died before his father, in the year 760 AD. 
- ↑ Meri, Josef W. "Medieval Islamic Civilization: An Encyclopedia". Routledge, NY. 2005, p 409 ISBN 978-0-41-596690-0
- ↑ A short bio of the sixth Imam at Rafed.Net
- ↑ Information about his children at al-Shia.ORG
- ↑ Scholarly Jihad of the Sixth Holy Imam at ImamReza.Net
- ↑ M Ali Amir Moezzi, The Divine Guide in Early Shi'sm : The Sources of Esotericism in Islam. edition=1st ed., State University of New York Press,1994, pg 108 isbn=978-0-79-142122-2
- ↑ Bio of the sixth Imam » Kitab al-Irshad by Shaykh al-Mufid
- ↑ Hodgson, Marshall G.S. "The Venture of Islam Volume 1: The Classical Age of Islam", The University of Chicago Press, USA, 1977, p. 262, ISBN 978-0-22-634683-0
- ↑ Richard C. Martin et al., Encyclopedia of Islam & the Muslim World, Macmillan Reference USA, 2003. pg 369 ISBN 978-0-02-865603-8
- ↑ Jafri, S.H Mohammad. "The Origin and Early Development of Shi'a Islam,”, Oxford University Press, 2002, p. 211, ISBN 978-0-19-579387-1
- ↑ Imam al-Sadiq bio at Maaref-Foundation
- ↑ Gleave, Robert. Scripturalist Islam: The History and Doctrines of the Akhbari Shia School, Brill, 2007, pp 18 ISBN 978-9-00-415728-6
- ↑ Philip K. Hitti, History of the Arabs: From the Earliest Times to the Present. Macmillan & Co. Ltd., 7th edition, 1961