This document summarizes several models of second language acquisition (SLA) and discusses how they account for individual differences in language learning outcomes. It describes models such as the Good Language Learner model, Monitor Theory, Brown and Fraser's framework, Levin's schematic model, and Gardner's educational model. The document also discusses several individual variables that can influence SLA, such as age, attitude, motivation, intelligence, learning aptitude, and previous knowledge. It concludes that while study findings are not entirely consistent, most research indicates individual-related variables significantly impact language learning achievement.