Tadlis is one of the defects that weaken Hadilh, and if Hadith is affected by Tadlis it is the one that is termed by Muhaddithun as Mudallas, and it is very difficult to trace. Because there is no enough books particularly on Tadlis, especially in English Language, while Mudallas is one of the most dangerous types of weak Hadith. Therefore, this research was undertaken to study the concept of Tadlis with regard to the science of Hadith. The clear meaning of Tadlis, its types, categories and the rule governing each category has been discussed. The characteristics of Mudallisun, their classes and the grade of their narrations have been analyzed. It was also explained that mistakenly Tadlis is attributed to some famous scholars, which the study confirmed it wrong to accuse such scholars of Practicing Tadlis, hence purifies them from any hidden defect not to talk on clear defect like Tadlis. The recommended commentaries of the glorious Qur'an, books of the Science of Hadith, critical judgment of specialists in the field of the science of Hadith, and their remarks were used as guide in reviewing, checking and scrutinizing the collected data, which was presented in analyzing the concept. The researcher succeeded in finding out that the issues of Tadlis, Mudallisun and Mudallas in the science of Hadith are very sensitive, which need accuracy, carefulness and patience in studying, because there are real and relative and each needs to be treated exclusively. It has also succeeded in bringing out, that Tadlis has different meanings and types with several forms, and each type or form has its own judgment particularly, and every Mudallis should be treated individually. Conclusion and recommendations were made.