The most important method of getting the knowledge of "Ahadith" is Isma-ul- Rajal. It was started in the reign of holy followers. As Holy Prophet's followers were willing to believe all the sayings of Holy Prophet. After the holy followers, tabeen were also very conscious about the authenticity of Hadith. But the real criticism about the authenticity of Hadith started in the 3rd (C. A.H). It was in this age that many books were written on the life if different narrators. Books about weak narrators were also written. In this way Ummah was given awareness about the fake Hadits. Hence it became possible to differentiate between correct and incorrect Hadits. This article deals with such books. We can have a glimpse of such books in this article.