Dr. Krashen has published more than 350 papers and books about second-language acquisition, bilingual education, and reading. He introduced several important ideas in the study of learning another language, including the acquisition-learning hypothesis, input hypothesis, monitor hypothesis, affective filter, and the natural order hypothesis. He started the Natural Approach to language learning and teaching with Tracy D. Terrell . More 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."