Language is crucial for adapting to society, and learning different languages enhances human efficiency. Learning a second language involves acquiring any language in addition to one's native language. Second Language Acquisition (SLA) has two meanings, generally, it describes the process of learning a second language, and more specifically, it refers to the theory of how a second language is acquired or picked up. Second language learning demands considerable effort and practice. Stephen Krashen, a prominent linguist, has conducted extensive research on non-English and bilingual language acquisition. Krashen's theory of second language learning is founded on five major hypotheses, the Acquisition-Learning Hypothesis, the Natural Order Hypothesis, the Monitor Hypothesis, the Input Hypothesis, and the Affective Filter Hypothesis. Given its broad scope and significant impact on language teaching approaches, Krashen's Second Language Acquisition theory is better described as a theory rather than merely a hypothesis. However, like all scientific theories, it continues to be refined and debated in scholarly circles. This article explores Krashen's theories of Second Language Acquisition and provides justifications for referring to them as theories.