The Critical Period Hypothesis in Second Language Acquisition posits that there is a critical period, early childhood until puberty, in which human beings must acquire a second language if they are going to achieve native-like attainment in that language. This thesis is a review of the current state of research in regards to the Critical Period Hypothesis in Second Language Acquisition. While evidence is provided to refute the Critical Period Hypothesis in the studies examined, a general age effect is found in the native-like attainment of a second language both in the acquisition of grammatical features and in the acquisition of phonological system. A look at how to relate these findings to foreign language education in the United States is begun by looking specifically at foreign language immersion programs in the United States and their effectiveness on native-like attainment and proficiency. There are currently 448 foreign language immersion programs in the U.S. with 45% of these programs being Spanish immersion and 22% being French immersion. Research shows that foreign language immersion programs are effective in helping children acquire a second language more effectively, specifically the early immersion programs, as well as acquire more metalinguistic awareness of languages than their monolingual counterparts
