A fundamental principle of second language acquisition is stated and applied to language teaching. The principle states that learners acquire a second language when they receive comprehensible input in situations where their affective filters are sufficiently low. The theoretical background of this principle consists of five hypotheses: the acquisition-learning, natural order, monitor, input, and affective filter hypotheses. The fundamental principle helps to resolve several problems in the professional literature, including the effectiveness of second language instruction and the effects of age on second language acquisition. The principal applied to pedagogy posits that any instructional technique that promotes second language acquisition does so by providing comprehensible input. This pedagogical principle can be applied to a variety of language teaching methods and settings, including the language classroom, the media, the language laboratory, subject matter teaching, and immersion programs. (RW)