Ads that read, "If you can speak it, you can teach it," attracted thousands of young people to travel the world to teach English as a Second Language (ESL). English language schools of the 1960s and 1970s flourished with the influx of native speakers who were given a scripted textbook with all the "right" things to say. However, by the end of the 1970s, in a world that was rapidly globalizing, the need to communicate across language barriers had become critical. This would lead to the development of a completely new field of research that would change the way second-language classrooms operated, and to a large degree, the way students learned a new language. This article examines some of the more salient aspects of research that have influenced the understanding of what language is and how a learner acquires a second language.