Working memory is a cognitive device for simultaneous information storage and processing in ongoing learning tasks or events. Current theories of second language acquisition (SLA) posit a central role of interactive tasks in facilitating the process and outcome of second language development. The mechanism of working memory is a perfect match for the mechanism of learning that occurs in second language (L2) interaction. This chapter seeks to synthesize the literature on the relationship between working memory and L2 interaction including theoretical models and findings of empirical research. To achieve these objectives, the chapter starts with a discussion of major SLA theories that elaborate the relevance of working memory to L2 interaction including the Interaction Hypothesis (Long, 1996), the Cognition Hypothesis (Robinson, 2011), and the Limited Attentional Capacity Theory (Skehan, 1998). The chapter proceeds to review the various streams of research investigating the associations between working memory and (1) interactional behaviors, (2) task performance represented by the complexity, accuracy, and fluency of L2 speech, and (3) interaction-driven L2 gains. The chapter then concludes by identifying the strengths and limitations of existing research and recommending ways to address issues and advance the field.