Some people say that if you are a native speaker of English (or any language), you can teach it to others. Other people suggest that this sentiment is similar to stating that you can perform surgery if you know how to slice bread. This course explores the decades-long research on how people actually learn languages other than their first one(s), including the roles of identity, society, immigration, age, gender, instruction, and biology in the process. Undergraduate and graduate students in education, linguistics, communication, foreign languages, sociology, psychology, international studies, policy-making, or anyone who wants to know the science behind the development of competence in an additional language will find this course useful.