This paper attempts to introduce the dimensions of bi/multi-lingualism as a cognitive linguis-tic phenomenon, which can be thought over in sociological, biolinguistic, cultural and psy-cholinguistic perspective. The main research question being asked as how bi/multilingualism is a cognitive phenomenon and what are the bilinguality problems and how do they repre-sents in linguistics? Linguistic interference, bilingual acquisition, and mental lexicons notions and multi lingual representation in the brain are the main problems of the subject. There are, at least, two dif-ferent types of linguistic transfer, positive and negative or interference. The mental lexicon is the constituent of the grammar consisting of phonological, morphological, semantic, syntac-tic, and pragmatics information which speakers have stored about individual words and morphemes in their minds.The paper, therefore, is an attempt to answer and deschibe the above questions.