This paper discusses the definition of mother tongue and some relevant questions. It argues , after an investigation of the
monolingual
community, bilingual society, bilingual families, language shift, language extinction and other complexities, that mother tongue refers to the common language of a nation rather than dialects which should be regarded as native speech. First language and mother tongue are overlapping concepts. It is the language identity of a nation or an
individual that determines the mother tongue, but not the sequence of acquisition. Under special circumstances, there will appear phenomena of dual mother tongues and mother tongue loss. As it belongs to individuals as well as to a nation, mother tongue is the human rights that cannot be deprived of, however, any individual or community has the right to give up his or their mother tongue and to use other language (s) . Mother tongue is, in no way, always the natural means of thinking and communication or of expressing oneself, therefore, mother tongue education should not be mechanically and simply emphasized. Pedagogical language is chosen with the decision made by children' s parents or their guardians or a national community.