Investigate Critical-Period Hypothesis Ortega (2013), in chapter two, discussed that the acquisition of any language is closely connected to a particular period of time. The earlier you are exposed to a language, the more fluent you will be in that language. In other words, it is believed that children can easily acquire a language before the age of puberty. Researchers and linguists call this period the critical, optimal or sensitive period (Ortega ,2013). Therefore, I was eager to investigate this period of time, which is Critical Period, and there is research done by Penfield and Roberts (1959) and Lenneberg (1967) that proved the Critical-Period Hypothesis. However, there were other studies, such as the study that was done by Loup and her colleagues (1994) that disproved the CPH because Loup and her colleagues investigated a British woman who learned Arabic and was passed the critical period, but actually she became Native-Like in two and half years (as cited in Ortega, 2013, p.14-15). Research on the critical period is very important because this topic has impacted early childhood improvement and linguistics. To make it clear, I set out to provide evidence for this hypothesis. Hence, I investigated two articles that examined how language development is affected once learners have passed the critical period by comparing the articles and discussing the key findings to be used in teaching and learning strategies. The comparison of studies Current studies have examined the Critical-Period Hypothesis by looking at testing age effects on second language acquisition and the age of arrival to the United States. Utilizing the Chinese and Spanish speakers for their research showed reliability because both languages are structurally different than the English language. Indeed, both studies used samples of second language learners who varied in age; though, the study that was conducted by Hakuta, Bialystok, and Wiley (2003) had a larger number of immigrants. Hakuta et al (2003) used ages 15 and 20 on purpose. The age 15 was tested because it is associated with puberty, and 20 was used to test if discontinuity was presented after puberty. Therefore, the result of testing the critical period showed that there was a huge number of Chinese and Spanish speakers whose critical period ended at age 15. However, there were many of them whose critical period ended at age 20. Thus, after examining 15 and 20-year-olds, which means after the critical period, there is evidence that the English language proficiency depends on how long the learners spend on learning the language. The study claimed a clear discontinuity in the hypothesis that learning at a specific age would have indicated that the discontinuity in L2 acquisition essential to CPH is nonexistent (Hakuta, Bialystok and Wiley,2003).On the other hand, the study which was conducted by Huang (2014) investigated specifically if there is a significant and independent Age of Arrival in the United States effect (AOA) on L2 grammar and speech production. Huang (2014) used various ages starting from the age of 5 to ensure that participants were learning English as a second language, not both the languages Chinese and English. By providing grammar and speech tasks. The study’s results support the multiple critical period effects which means that there are CP of some areas of acquisition of second language end earlier than others, for example, the critical period for phonology, because some researchers, such as Long 2005; Newport et al. 2001; Werker and Tees 2005 demonstrated that critical period of phonology ended before puberty (as cited in Huang. pp. 413, 2014). The author showed that there is a specific age of learning effect for both L2 speech production and grammar outcomes, and the AoA had a larger influence on speech production than the grammar outcomes (Huang ,2014).Furthermore, these studies explored the CPH by looking into data from two perspectives: the rate of acquisition and ultimate attainment. In fact, Hakuta and his colleagues (2003) agreed that there is no essential part of the Critical-Period; however, they mentioned in their study that natural language acquisition is valuable to young children, but it was slightly limited in older adolescents and adults because they found a slow decline in the L2 acquisition. Both studies confirmed that when the L2 learners had a higher education and adequate time for residency and learning the language, they accomplished the native-like language successfully.The reflection of studies I learned how long it would take to find the specific articles to make a good connection because CPH is a long debate among linguists and language researchers, and it should be redefined for second language acquisition. A recent study by Huang (2014) argued with the studies that have been done by Penfield and Roberts (1959) and Lenneberg (1967) which confirmed the Critical-Period Hypothesis (CPH). In fact, Hakuta et al (2003) proved that there is a specific age of the critical period. Criticality, a self- report measure that was done by Hakuta et al (2003) could potentially lead to inaccuracies even though they used a large sample of bilingual immigrants to make their study strong. In fact, after investigating these articles, I believe that adults can acquire the L2 successfully no matter what are their ages. I am a second language learner and exploring this topic, indeed, answered many questions that I have faced since I started learning the language such as speech production which is related to phenology term. Importantly, as a second language teacher, being knowledgeable of the critical period is helpful because the findings can be used in teaching and learning strategies for language. Thus, the teacher will be conscious of the students’ capacity to acquire the SLA. For example, pronunciation is always the most difficult part of the second language learning; therefore, teachers should try to provide adequate materials that support the second language learners to handle their complexities in phonology terms because some studies done by Scovel and Singleton (1989) have debated that the phonology had the greatest impact after passing the CP (as cited in Huang pp, 399, 2014). Teachers should immerse the students in real life events of the language because adults can achieve native-like if they used the real-life situations. For instance, communication with native speakers can help learners of a second language to develop the native-like accent of that language gradually.