The Interaction hypothesis is a theory of second-language acquisition which states that the development of language proficiency is promoted by face-to-face interaction and communication. The idea existed in the 1980s, but is usually credited to Michael Long for his 1996 paper The role of the linguistic environment in second language acquisition. There are two forms of the Interaction Hypothesis: the "strong" form and the "weak" form. The "strong" form is the position that the interaction itself contributes to language development. The "weak" form is the position that interaction is simply the way that learners find learning opportunities, whether or not they make productive use of them.
Similarly to Krashen's input hypothesis, the interaction hypothesis claims that comprehensible input is important for language learning. In addition, it claims that the effectiveness of comprehensible input is greatly increased when learners have to negotiate for meaning. This occurs when there is a breakdown in communication which interlocutors attempt to overcome. One of the participants in a conversation will say something that the other does not understand; the participants will then use various communicative strategies to help the interaction progress. The strategies used when negotiating meaning may include slowing down speech, speaking more deliberately, requests for clarification or repair of speech, or paraphrases.
Interactions often result in learners receiving negative evidence. That is, if learners say something that their interlocutors do not understand, after negotiation the interlocutors may model the correct language form. In doing this, learners can receive feedback on their production and on grammar that they have not yet mastered. The process of interaction may also result in learners receiving more input from their interlocutors than they would otherwise. Furthermore, if learners stop to clarify things that they do not understand, they may have more time to process the input they receive. This can lead to better understanding and possibly the acquisition of new language forms. Finally, interactions may serve as a way of focusing learners' attention on a difference between their knowledge of the target language and the reality of what they are hearing; it may also focus their attention on a part of the target language of which they are not yet aware.
Primacy of interaction 
Although there are several studies that link interaction with language acquisition, not all researchers subscribe to idea that interaction is the primary means by which language proficiency develops. In a survey of the literature on the subject, Larsen-Freeman and Long say that interaction is not necessary for language acquisition; they do say, however, that it helps in certain circumstances. Gass and Selinker claim that as well as interaction facilitating learning, it may also function as a priming device, "setting the stage" for learning rather than being the means by which learning takes place. In addition, Ellis notes that interaction is not always positive. He says that sometimes it can make the input more complicated, or produce amounts of input which overwhelm learners. According to Ellis, this can happen if interlocutors use lengthy paraphrases or give complex definitions of a word that was not understood, and he comes to the conclusion that the role of interaction in language acquisition is a complex one.
- Johnson, Keith; Johnson, Helen, eds. (1999). "Interaction Hypothesis". Encyclopedic Dictionary of Applied Linguistics: A Handbook for Language Teaching. Oxford: Blackwell Publishers. p. 174. ISBN 978-0-631-22767-0.
- Long, Michael (1985). "Input and Second Language Acquisition Theory". In Gass, Susan; Madden, Carolyn. Input in second language acquisition. Rowley, Mass: Newbury House. pp. 377–393. ISBN 978-0-88377-284-3.
- Ellis, Rod (1984). Classroom Second Language Development: A Study of Classroom Interaction and Language Acquisition. Oxford, UK: Pergamon. p. 95. ISBN 978-0-08-031516-4.
- Long, Michael (1996). "The role of the linguistic environment in second language acquisition". In Ritchie, William; Bhatia, Tej. Handbook of second language acquisition. San Diego: Academic Press. pp. 413–468. ISBN 978-0-12-589042-7.
- Ellis, Rod (1997). Second Language Acquisition. Oxford Introductions to Language Study. Oxford, New York: Oxford University Press. pp. 47–48. ISBN 978-0-19-437212-1.
- Richards, Jack; Schmidt, Richard, eds. (2002). "Interaction Hypothesis". Longman dictionary of language teaching and applied linguistics. London New York: Longman. p. 264. ISBN 978-0-582-43825-5.
- Brown, H Douglas (2000). Principles of Language Learning and Teaching. White Plains, NY: Longman. pp. 287–288. ISBN 978-0-13-017816-9.
- For an overview, see Gass, Susan; Selinker, Larry (2008). Second Language Acquisition: An Introductory Course. New York, NY: Routledge. pp. 353–355. ISBN 978-0-8058-5497-8.
- Larsen-Freeman, Diane; Long, Michael (1991). An Introduction to Second Language Acquisition Research. London, New York: Longman. pp. 143–144. ISBN 978-0-582-55377-4.