In Twelver Shia Islam, the Major Occultation refers to the longer duration of the Occultation of the final and twelfth Imam, Muhammad al-Mahdi, continuing to the present day. According to the last letter of Muhammad al-Mahdi to Ali ibn Muhammad al-Samarri "from the day of your death [the last deputy] the period of my major occultation (al ghaybatul kubra) will begin. Hence forth, no one will see me, unless and until Allah makes me appear. My reappearance will take place after a very long time when people will have grown tired of waiting and those who are weak in their faith will say: What! Is he still alive?"
Another letter from Muhammad al-Mahdi says: "Rest assured, no one has a special relationship with Allah. Whoever denies me is not from my (community). The appearance of the Relief (al-Faraj) depends solely upon Allah. Therefore those who propose a certain time for it are liars. As to the benefit of my existence in occultation, it is like the benefit of the sun behind the clouds where the eyes do not see it." - Usul Al-Kafi, Ibn Kulayni
With regards to advice for his followers during his absence, he is reported to have said: "Refer to the transmitters of our traditions, for they are my hujja (proof) unto you and I am God’s proof unto them."