This paper will examine the interaction hypothesis (IH) in second language acquisition (SLA). To begin with a short discussion of the confusing terms in SLA such as theory, model, hypothesis, and construct will be done so as to help readers easily understand theories in the field of SLA and related concepts. Next, what the IH is, and who proposed it will be discussed in detail. How the IH has evolved and has been modified since its inception will then be pointed out. The origins of the IH will also be discussed. In addition, the role of the IH in the field of SLA will be presented together with its positive contributions as well as its caveats. Research studies that support the hypothesis and those that do not support it will both be listed. Moreover, all the constructs in the IH will be delineated. The implications that the hypothesis has for SLA pedagogical research will also be mentioned. Overall, the hypothesis can probably be considered as one of the most persuasive in current SLA literature.