The promised Mahdi, who is usually mentioned by his title of Imam-i 'Asr (the Imam of the "Period") and Sahib al-Zaman (the Lord of the Age), is the son of the eleventh Imam. His name is the same as that of the Holy Prophet. He was born in Samarrah in 256/868 and until 260/872 when his father was martyred, lived under his father’s care and tutelage. He was hidden from public view and only a few of the elite among the Shi'ah were able to meet him.
After the martyrdom of his father he became Imam and by Divine Command went into occultation (ghaybat). Thereafter he appeared only to his deputies (na'ib) and even then only in exceptional circumstances.
The Imam chose as a special deputy for a time Uthman ibn Sa'id 'Umari, one of the companions of his father and grandfather who was his confidant and trusted friend. Through his deputy the Imam would answer the demands and Questions of the Shi'ah.
After Uthman ibn Sa'id, his son Muhammad ibn Uthman Umari was appointed the deputy of the Imam. After the death of Muhammad ibn Uthman, Abu'l Qasim Husalyn ibn Ruh Nawbakhti was the special deputy, and after his death Ali ibn Muhammad Simmari was chosen for this task.
A few days before the death of Ali ibn Muhammad Simmari in 329/939 an order was issued by the Imam stating that in six days Ali ibn Muhammad Simmari would die, Henceforth the special deputation of the Imam would come to an end and the major occultation (ghaybat-i kubra) would begin and would continue until the day God grants permission to the Imam to manifest himself.
The occultation of the twelfth Imam is, therefore, divided into two parts: the first, the minor occultation (ghaybat-i sughra) which began in 260/872 and ended in 329/939, lasting about seventy years; the second, the major occultation which commenced in 329/939 and will continue as long as God wills it. In a hadith upon whose authenticity everyone agrees, the Holy Prophet has said, "If there were to remain in the life of the world but one day, God would prolong that day until He sends in it a man from my community and my household, His name will be the same as my name.
He will fill the earth with equity and justice as it was filled with oppression and tyranny."

On the Appearance of the Mahdi
In the discussion on prophecy and the imamate it was indicated that as a result of the law of general guidance which governs all of creation, man is of necessity endowed with the power of receiving revelation through prophecy, which directs him toward the perfection of the human norm and the well-being of the human species. Obviously, if this perfection and happiness were not possible for man, whose life possesses a social aspect, the very fact that he is endowed with this power would be meaningless and futile. But there is no futility in creation.
In other words, ever since he has inhabited the earth, man has had the wish to lead a social life filled with happiness in its true sense and has striven toward this end. If such a wish were not to have an objective existence it would never have been imprinted upon man's inner nature, in the same way that if there were no food there would have been no hunger. Or if there were to be no water there would be no thirst and if there were to be no reproduction there would have been no sexual attraction between the sexes.
Therefore, by reason of inner necessity and determination, the future will see a day when human society will be replete with justice and when all will live in peace and tranquillity, when human beings will be fully possessed of virtue and perfection. The establishment of such a condition will occur through human hands but with Divine succor. And the leader of such a society, who will be the savior of man, is called in the language of the hadith, the Mahdi.
In the different religions that govern the world such as Hinduism, Buddhism, Judaism, Christianity, Zoroastrianism and Islam there are references to a person who will come as the savior of mankind. These religions have usually given happy tidings of his coming, although there are naturally certain differences in detail that can be discerned when these teachings are compared carefully. The hadith of the Holy Prophet upon which all Muslims agree; 'The Mahdi is of my progeny," refers to this same truth.
There are numerous hadiths cited in Sunni and Shi'ite sources from the Holy Prophet and the Imams concerning the appearance of the Mahdi, Such as that he is of the progeny of the Prophet and that his appearance will enable human society to reach true perfection and the full realization of spiritual life.
In addition, there are numerous other traditions concerning the fact that the Mahdi is the son of the eleventh Imam, Hasan al-'Askari. They agree that after being born and undergoing a long occultation the Mahdi will appear again, filling with.
Justice the world that has been corrupted by injustice and iniquity.
As an example, Ali ibn Musa al-Rida (the eighth Imam) has said, in the course of a hadith, "The imam after me is my son, Muhammad and after him his son Ali, and after Ali his son, Hasan, and after Hasan his son Hujjat al-Qa'im, who is awaited during his occultation and obeyed during his manifestation. If there remain from the life of the world but a single day, Allah will extend that day until he becomes manifest, and fill the world with justice in the same way that it had been filled with iniquity. But when? As for news of the 'hour,' verily my father told me, having heard it from his father who heard it from his father who heard it from his ancestors who heard it from Ali, that it was asked of the Holy Prophet, 'Oh Prophet of God when will the 'support' (qa'im) who is from thy family appear?' He said. 'His case is like that of the Hour (of the Resurrection). "He alone will manifest it at its proper time. It is heavy in the heavens and the earth. It cometh not to you save unawares'. (Quran. VII, l87).’"
Saqr ibn Abi Dulaf said. "I heard from Abu Ja'far Muhammad ibn Ali al-Rida [the ninth Imam] who said, 'The Imam after me is my son. Ali; his command is my command; his word is my word ; to obey him is to obey me. The Imam after him is his son. Hasan. His command is the command of his father his word is the word of h is father to obey him is to obey his father.' After these words the Imam remained silent. I said to him, 'Oh son of the Prophet who will be the Imam after Hasan?' The Imam cried hard, then said.
'Verily after Hasan his son is the awaited Imam who is "al-qa'im bi'l-haqq'. (He who is supported by the Truth),'"
Musa ibn Ja'far Baghdadi said, "I heard from the Imam Abu Muhammad al-Hasan ibn Ali [the eleventh Imam] who said, 'I see that after me differences will appear among you concerning the Imam after me. Whose accepts the Imams after the Prophet of God but denies my son is like the person who accepts all the prophets but denies the prophethood of Muhammad, the Prophet of God upon whom be peace and blessing. And whose denies [Muhammad] the Prophet of God is like one who has denied all the prophets of God, for to obey the last of us is like obeying the first and to deny the last of us is like denying the first. But beware!
Verily for my son there is an occultation during which all people will fall into doubt except those whom Allah protects."
The opponents of Shi'ism protest that' according to the beliefs of this school the Hidden Imam should by now be nearly twelve centuries old, whereas this is impossible for any human being. In answer it must be said that the protest is based only on the unlikelihood of such an occurrence not its impossibility. Of course such a long lifetime or a life of a longer period is unlikely. But those who study the hadiths of the Holy Prophet and the Imams will see that they refer to this life as one possessing miraculous qualities.
Miracles are certainly not impossible nor can they be negated through scientific arguments. It can never be proved that the causes and agents that are functioning in the world are solely those that we see and know and that other causes which we do not know or whose effects and actions we have not seen nor understood do not exist. It is in this way possible that in one or several members of mankind there can be operating certain causes and agents which bestow upon them a very long life of a thousand or several thousand years. Medicine has not even lost hope of discovering a way to achieve very long life spans. In any case such protests from "peoples of the Book" such as Jews, Christians and Muslims are most strange for they accept the miracles of the prophets of God according to their own sacred scriptures.
The opponents of Shi'ism also protest that, although Shi'ism considers the Imam necessary in order to expound the injunctions and verities of religion and to guide the people, the occultation of the Imam is the negation of this very purpose, for an Imam in occultation who cannot be reached by mankind cannot be in any way beneficial or effective.
The opponents say that if God wills to bring forth an Imam to reform mankind He is able to create him at the necessary moment and does not need to create him thousands of years earlier. In answer it must be said that such people have not really understood the meaning of the Imam, for in the discussion on the imamate it became clear that the duty of the Imam is not only the formal explanation of the religious sciences and exoteric guidance of the people. In the same way that he has the duty of guiding men outwardly, the Imam also bears the function of walayat and the esoteric guidance of men. It is he who directs man's spiritual life and orients the inner aspect of human action toward God. Clearly, his physical presence or absence has no effect in this matter. The Imam watches over men inwardly and is in communion with the soul and spirit of men even if he be hidden from their physical eyes. His existence is always necessary even if the time has not as yet arrived for his outward appearance and the universal reconstruction that he is to bring about.