This document summarizes Stephen Krashen's theory of second language acquisition, which formed the basis for bilingual education in the US. The theory consists of 5 main hypotheses: 1) Acquisition-Learning, 2) Monitor, 3) Natural Order, 4) Input, and 5) Affective Filter. It explains that acquisition is subconscious while learning is conscious, the role of monitoring, that structures are acquired in a predictable order, comprehensible input drives acquisition, and affective variables like anxiety and motivation impact the learning process.