Learning a second or foreign language is a long and time-consuming process, and not all language learners may be able to achieve a very high level of proficiency in the target language. It is even rarer to find second or foreign language learners who can use the target language as well as native speakers of the language. Researchers in the field of second language acquisition (SLA) have long been concerned with the factors that prevent learners from attaining such a successful level of language learning. Among various factors that have been empirically investigated, age has seemed to figure prominently in the literature. The critical period hypothesis (CPH) has been proposed to explicate the reason why adult language learners are unable to achieve native-like proficiency. This paper aims at providing a brief overview of the CPH. First, the origin of the CPH is pointed out. Second, studies that support the CPH and those that attempt to falsify the CPH are discussed respectively. It is argued that although it is hard for adult learners to attain native-like proficiency in another language, it may still be possible. Moreover, the importance of finding out if the CPH is true or not may not be as important as trying to help learners to use the language more effectively. It is concluded that helping learners to communicate effectively in the target language is more logically and realistic than trying to help them to emulate native speakers of the target language.