Dr. Stephen Krashen is professor emeritus at the University of Southern California, who moved from the linguistics department to the faculty of the School of Education in 1994. He is a linguist, educational researcher, and activist.
Dr. Krashen has published more than 350 papers and books, contributing to the fields of second language acquisition, bilingual education, and reading. He is credited with introducing various influential concepts and terms in the study of second language acquisition, including the acquisition-learning hypothesis, the input hypothesis, the monitor hypothesis, the affective filter, and the natural order hypothesis. Most recently, Krashen promotes the use of free voluntary reading during second language acquisition, which he says "is the most powerful tool we have in language education, first and second."
(Bio from Wikipedia)