Language is very important for human being to adopt himself in society and learning different languages increases human efficiency. Learning a Second language refers to learning of any language in addition to the native language. Second language acquisition, or SLA, has two meanings. In a general sense it is a term to describe learning a second language. More specifically, it is the name of the theory of the process by which we acquire - or pick up - a second language. Second language learning requires lot of efforts and practice. Stephen Krashen is an expert in the field of linguistics, and his research has involved the study of non-English and bilingual language acquisition. Krashen’s theory of Second language learning is based on five major hypotheses namely a) The acquisition-learning hypothesis, b) The natural order hypothesis, c) The monitor hypothesis, d) The input hypothesis, and e) The affective-filtering hypothesis. Because of its extensive scope and profound influence on language teaching approaches, Krashens Second Language Acquisition is better described as a theory than a hypothesis. However, it should be noted that, like all scientific theories, it is still being improved upon and discussed in scholarly circles. The theories of Krashen's Second Language Acquisition are covered in this article, along with the justifications for calling them theories.