This paper is an introduction is to Stephen Krashen's five Language Acquisition Hypotheses, which serve as the basis of his development of the Natural Approach Method of communicative language teacing. The hypotheses were examined individually in order to a) define the general theory, b) provide the merits of each theory in terms of language teaching, and c) the problems or difficulties as observed by both linguists and language teachers. Each of the hypotheses was then considered from the viewpoint of English language education in Japan, to determine whether this context might support or deny the validity and/or usefulness of each theory. In conclusion, it was determined that although Krashen's hypotheses have merit as tools which promote the overall consideration of methods of language acquisition and teaching, they also have serious deficiencies, both in their development (lack of sufficient evidence, unclear terminology, etc.) and in their application (lack of consideration of the cultural background of the learner, lack of emphasis on productive activities, etc.).