This document discusses interlanguage and the natural route of second language acquisition. It defines interlanguage as the systematic language knowledge that is independent of both the learner's first language and the target second language. Interlanguage refers to the structured system learners construct at each stage of development, as well as the series of interconnected systems that form the learner's "built-in syllabus". The natural route of second language acquisition follows broad stages of development similar to first language acquisition, though the order may differ due to factors like the first language. Input and interaction play an important role in second language acquisition.