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. Its main focus is on the role of input, interaction, and output in second language acquisition. It posits that the level of language that a learner is exposed to must be such that the learner is able to comprehend it, and that a learner modifying their speech so as to make it comprehensible facilitates their ability to acquire the language in question. The idea existed in the 1980s, and has been reviewed and expanded upon by a number of other scholars but is usually credited to Michael Long.