Given the fact that there is a constant debate among monolinguists and pluralists, this chapter aims to explore the main developments in learner language. It focuses on the changes from second language research to learner corpus research. It is an attempt to present second language theories. Then, the chapter draws a particular attention to the limitations of second language acquisition. The discussion turns to learner corpus research to show how language changes from heterogeneinity to diversity. Language is no longer seen as monolithic entity or a standard variety but a multilingual entity.