The type of language constructed by second or foreign language learners who are still in the process of learning a language is often referred to as interlanguage. Interlanguage which preserves some features of their first language (or L1), and can also over generalize some L2 writing and speaking rules is a language system between the target language and the learner's native language. It's imperfect compared with the target language, but it is not mere translation from the learner's native language. Interlanguage is a dynamic system, which is constantly moving from the departure level to the native-level. Studies on interlanguage can be done in two ways: 1) investigating the psychological, biological or neurological mechanisms involved in the production of interlanguage; 2) investigating the linguistic features of interlanguage.