Stephen Krashen is an American linguist and educator whose name became almost synonymous with the great input turn in late twentieth-century SLA. Born in Chicago in 1941, he earned his doctorate in linguistics at UCLA and went on to teach at the University of Southern California, where he later became Professor Emeritus of Education.
Krashen's public career has had two strikingly different registers. In academic work, he is the author most closely associated with the Monitor Model, Comprehensible Input, the Input Hypothesis, and the Affective Filter. In public argument, he has also been a tireless advocate for bilingual education, literacy, and free voluntary reading. He writes with the assurance of someone who has been debated for decades and knows perfectly well that being challenged is part of the job.