Now it is about 12 centuries passed from Imam Mahdi's hidden life, and the bothering time of his hidden life will continue up to his reappearance. According to imam Ridha (as), Imam's reappearance will be extended to the day of Doom, no one knows this but God, it is hard to people and it will happen suddenly. Therefore, any investigation to find the exact time of his reappearance is fruitless and our duty is to wait. The meaning of "waiting for" is to desire his reappearance desperately and looking forward to his reappearance; this waiting results from faith and it is rewarded. In addition, it has spiritual values. Waiting for him, like other religious practices, has practical aspects which is pointed out by our infallibles. Following infallibles' orders in this regard is the duty of people who live in the time of his hidden life. The book deals with the personality of the Hidden Imam, his occultation and everything related to him in an analytical and systematic manner and is penetrating in its subject and exceptional in its domain. Shaikh Saduq has presented strong arguments from the life-story of the Prophets regarding their occultation [ghaibah] to prove the occultation of the Twelfth Imam; and Ahadith have been quoted extensively from the Holy Prophet sawa and the Infallible Imams in support of the writer's contentions.