This paper describes the definitions of “Deaf/deaf” in English, which are from the medical and social views; also to describe that “sign language” is the Deaf mother tongue. It also describes the differences of foreign sign languages and the use of sign languages in Thailand including gesture, Home Sign, Sign Thai (sign as Thai speaking language) and Thai Sign Language; and the variation of Thai Sign Language which is used differently according to regional, gender and age factors. These differences caused the problems for Thai Sign Language more than spoken languages and maybe more than the foreign sign languages. However these problems can be improved and solved by pushing forward to establish the government policy and every organization concerned to encourage the Deaf children, parents, and interpreters to use Thai Sign Language correctly.