The Interaction Hypothesis,which was first advanced by Michael Long,is considered as a further supplement or extension of the Input Hypothesis in the field of second language acquisition. This paper explains the main claims of Michael Long’s Interaction Hypothesis,the three steps to study how input and interaction affect the acquisition of a second language,and the importance of the interactional modifications applied by both participants of the interaction. Emphasis is placed on the evaluation of the theory as well as its relation to second language teaching,with many researchers’ findings and arguments discussed as supporting evidence. It aims to help the effective application of this theory in second language teaching.