This document summarizes several theories of second language acquisition (SLA). It discusses Stephen Krashen's Acquisition-Learning Hypothesis, which distinguishes between acquired unconscious knowledge and learned conscious knowledge. It also covers the Natural Order Hypothesis, Monitor Hypothesis, Input Hypothesis, and Affective Filter Hypothesis. Additional topics include the role of language teaching, factors like exposure, age, and John Schumann's Acculturation Model that examines how social and psychological distance affect SLA. The document provides definitions and explanations of the key terms and hypotheses in SLA theory.