Interaction is an indispensable component in second language acquisition (SLA). This review surveys the instructed SLA research, both classroom and laboratory-based, that has been conducted primarily within the interactionist approach, beginning with the core constructs of interaction, namely input, negotiation for meaning, and output. The review continues with an overview of specific areas of interaction research. The first investigates interlocutor characteristics, including (a) first language (L1) status, (b) peer interaction, (c) participation structure, (d) second language (L2) proficiency, and (e) individual differences. The second topic is task characteristics, such as task conditions (e.g. information distribution, task goals), task complexity (i.e. simple or complex), and task participation structure (i.e. whole class, small groups or dyads). Next, the review considers various linguistic features that have been researched in relation to interaction and L2 learning. The review then continues with interactional contexts, focusing especially on research into computer-mediated interaction. The review ends with a consideration of methodological issues in interaction research, such as the merits of classroom and lab-based studies, and the various methods for measuring the noticing of linguistic forms during interaction. In sum, research has found interaction to be effective in promoting L2 development; however, there are numerous factors that impact its efficacy.