This chapter begins with an historical overview of theory and research on the role of learner interaction in the processes and sequences of second language acquisition (SLA). The overview highlights the foundational constructs of input, output, interaction, and comprehension, and current constructs of negotiation, attention, and evidence for SLA. The chapter also addresses the ways in which these constructs have illuminated the processes of SLA, shed light on the needs of the learner, and led to instructional approaches that facilitate effective second language (L2) outcomes. Examples of interaction-based approaches are provided. These include research validated strategies and tasks that provide opportunities for learners to interact in the L2 as they negotiate its meaning, attend to its linguistic forms and constructions, and access positive and negative evidence for their SLA