The document presents an overview of theories of second language acquisition (SLA). It discusses domains to consider in an SLA theory, including that SLA involves cognitive variations and is intertwined with culture learning. Several SLA theories are then summarized, including Krashen's Input Hypothesis with its sub-hypotheses, cognitive models focusing on attention and implicit/explicit knowledge, and Long's Interaction Hypothesis emphasizing the role of modified interaction between learners and native speakers. The theories represent innatist, cognitive, and social constructivist perspectives on SLA.