Stephen Krashen is professor emeritus at the University of Southern California, and is a linguist, educational researcher, and activist. Stephen Krashen has contributed to the fields of second language acquisition (SLA), bilingual education, and reading. He is credited with introducing various influential concepts and terms in the study of second language acquisition, including the distinction between acquisition and learning, the Input Hypothesis, Monitor Theory, the Affective Filter, and the Natural Order Hypothesis.