IntroductionThe critical period hypothesis has long been the subject of study. The question here is simple: does it really exist? And if it exists, to what extent does it influence second language proficiency? Many studies seem to have demonstrated that the critical hypothesis exists because apparently no adult has managed to achieve native-like skills after puberty. On the contrary, some believe that this hypothesis has no reason because some late learners have managed to achieve fluency equal to that of natives. There is obviously some decline in the ability to learn a second language with age. The reason for this remains controversial, as the discussion focuses on whether the cause is the critical period or other factors influencing second language learning. This article will discuss the perspective of the critical period hypothesis. Additionally, it will review studies for and against the critical period hypothesis. Each group can claim some evidence in its favor. The document will also focus on some factors that can influence second language proficiency; for example, age, use of the native language, and length of stay in an area where the second language is used. Definition of the Critical Period Hypothesis Snow, Catherine, and Hohle (1978) point out that "the critical period hypothesis holds that first language acquisition must occur before cerebral lateralization is complete, at approximately the age of puberty . One prediction of this hypothesis is that second language acquisition will be relatively fast, successful, and qualitatively similar to the first language only if it occurs before the age of puberty” (Snow, Catherine, & Hohle, 1978, p. 1114). cited in Snow, Catherine and Hohle (1978), L...... center of article ......rd University.Piske, T., MacKay, I. & Flege, J. (2001). “Factors influencing the degree of foreign accent in anL2: a review.” J. Phonetics 29, 191–215Snow, C. E., Hoefnagel-Höhle, M., & Hoefnagel-Höhle, M. (1978). language acquisition: Evidence from second language learning. Child Development, 49(4), 1114-1128.Tokudome, M. (2010).” Unlikely bedfellows: the critical period hypothesis and its effects on second language acquisition”. TPFLE, 14(1), 18-27Weber-Fox, C., & Neville, H. (1999). functional are differentially affected by delays in second language immersion: ERP and behavioral evidence in bilinguals. In D. Birdsong (Ed.), Second language acquisition and the critical period hypothesis (pp. 23–38). , NJ: Lawrence Erlbaum Associates.