Stephen Krashen is an American linguist and educational researcher known for his theories on second language acquisition. He developed the Input Hypothesis, which suggests that language learners acquire new languages best when they are exposed to language that is slightly above their current level of understanding. This idea emphasizes the importance of comprehensible input in learning. Krashen has also proposed the Affective Filter Hypothesis, which highlights the emotional factors that can influence language learning. According to this theory, a learner's motivation, anxiety, and self-confidence can either facilitate or hinder their ability to acquire a new language. His work has significantly impacted language education practices worldwide.