Isma'il ibn Jafar (Arabic: إسماعيل بن جعفر, c. 721 CE/103 AH - 755 CE/138AH) was the eldest son of the sixth Shia Imam, Jafar as-Sadiq. Isma'il bin Ja'far is considered to be the seventh Isma'ili Faatemi Imaam. He was born in Shawwal 100 AH/719 AD. To protect him from persecution, his father sent him into hiding and publicly declared him deceased. The majority Twelver group however argue that Ismail actually died, and that the funeral was not a fraud. He died during the Imamat of Imam Ja'far al- Sadiq in the year 138 AH (Appro .)756 AD.
Some sources indicate that the Imam Jafar as-Sadiq designated Ismail as his successor and the next Imam after him. However, it is controversially believed that Ismail died before his father. Therefore, upon as-Sadiq's death, some Shi’i followers turned to the eldest surviving son of al-Sadiq, Abd-Allah, who claimed a second designation. When Abd-Allah died within weeks without an heir, many turned again to another son of as-Sadiq, Musa al-Kazim, while some had already accepted him as the Imam previously. This crisis of succession led to doubts about the true designation of Jafar as-Sadiq and many clung to the original designation of Isma’il and proclaimed the son of Isma’il.
The Twelver school of Thought does not accept Ismail ibn Jafar, and instead believe Musa al-Kadhim was the successor. According to Twelver and Sunni sources, names of all 12 Imams were narrated by Muhammad. They claim that Muhammad stated that after Jafar ibn Muhammad, his son Musa ibn Jafar al-Kazim is the Seventh Imam. The minority who believe that Isma'il was the rightful successor to Jafar are known as Ismailis, while the other majority of followers of Musa Al-Kazim are known as Twelvers. They went on to recognize five more Imams after Musa - making twelve Imams in total. The Nizari Ismaili Shias follow a living Imam directly descended from Imam Isma'il ibn Jafar & Imam Ali ibn Abi Talib. The Ismaili Imams are today known as the Aga Khans - the present (hadir) Imam being Shah Karim al-Husayni - or Aga Khan IV.