The bilingual instrument, because of its greater complexity, is more difficult to master, but once mastered, has a greater potential than the unilingual instrument for promoting cognitive growth and communicative competence. The question of 'what are the possible linguistic and cognitive consequences of bilingualism?' has been a challenging area with controversial results among scholars since the last two decades. An attempt is made in the present paper to review the research concerning the possible cognitive and linguistic outcomes of bilingualism as related to additional language acquisition. Prior to 1960s researchers were primarily concerned with the effects of bilingualism and as a result of the consequential findings bilingualism was generally portrayed as a handicap (Saer, 1923). However, these conclusions were challenged by Peal and Lambert in 1962, and a majority of studies since then have reported a bilingual superiority on measures from various cognitive and linguistic areas. The studies reviewed in the present paper involve metalinguistic awareness (Jessner, 1999, Cenoz and Valencia, 1994, and Bild & Swain, 1989), cognitive flexibility (Riccardelli, 1992, Kozulin, 1988,) and third language acquisition (Cenoz & Jessner, 2000, Thomas, 1988). The results of these studies provide considerable evidence that the development of competence in two languages can result in greater levels of cognitive functioning and metalinguistic awareness, which in turn have a mediating effect on additional language acquisition. Bilingualism brings about new skills in the individual that enables him to manage the learning situations very competently. In interpersonal communications, the bilingual speaker has the ability to incorporate the language resources in his inventory and at the same time keep them apart. This accounts for the higher development of a communicative competence proposed by (Cenoz& Valencia, 1994) which is found to be more advanced among bilinguals than monolinguals. Metalinguistic awareness increased considerably among bilinguals was also viewed as another positive outcome of bilingualism. Bilinguals are thought to be more aware of how to reflect on their language use and usage, compare their language systems, thus, benefit from more learning strategies. These outcomes are consistent with the notion of switching hypothesis that has been proposed by various investigators (Balkan, 1970; Carringer, 1974; Landry, 1974; Peal and Lambert, 1962). In this review, there was a support for the hypothesis that far from making a hindrance in learning by the additional linguistic knowledge, bilingualism has a positive mediating effect on additional language learning. In other words, research findings manifested the idea that when one has already managed the difficult task of learning two languages, indeed he/she has developed a competence to cope with the new learning situations rather easier than his unilingual peers. This is consistent with Ringbom (1985) who suggests that "the structural knowledge of two languages could help to develop the bilinguals' metalinguistic awareness, as a consequence, to increase their ability in learning a third or even a forth language". The metalinguistic awareness could possibly account for the higher level of linguistic competence in third language learning. Finally, the cognitive outcomes of bilingualism were viewed in terms of increased cognitive flexibility and metalinguistic awareness evident among bilinguals. This superiority confirms Vygotsky's general theoretical analysis of language development and its relation to cognitive development. Furthermore, the results of the distinct experiments to bring about a synthesis between cognitive development and bilingualism carried out by various researchers like (Riccardelli, 1992 and Kozulin, 1998) go in line with the most findings in the area of bilingualism.