In spite of the fact that majority of Muslims, relying on religious and rational evidences, have a negative outlook toward hadith falsification and attributing false words and deeds to the Infallible Imams, peace be upon them, however their hadith sources show that they did not regard it seriously.The author of the present paper classifies the hadith forgers into two groups, one group are those who forge hadith with worldly purposes and the second group are those who do this with religious purposes and investigates their evidences.He also explains about some concepts concerning the phenomenon of forging hadith such as: definition of forged hadith; religious judgment on forging hadith; evidences of those who are in favor of forging hadith including good intention, guiding people, and administering justice.