A great deal of research on second language (L2) input has provided useful linguistic evidence for the development of generalizations about and hypotheses on the usage-based nature of L2 knowledge. However, despite the fact that classrooms are one of the most ubiquitous sites of L2 learning, we still know very little about the linguistic quality of naturally-occurring classroom interactions and their consequential role in shaping learners' linguistic repertoires (Ellis, 2016). These data are needed in order to understand how differences in L2 classroom interactional activities affect L2 learner language. This is the focus of the set of research tasks I lay out in this paper. After a brief overview of usage-based research on language and a summary of the methodological contributions of conversational analysis (CA) and interactional linguistics (IL) to such research, I will explicate a research agenda comprising five tasks for investigating the links between L2 classroom interaction and L2 learners' developing linguistic repertoires. By making clear the interrelationships between teaching and learning, findings from the studies will offer new insights into L2 pedagogy and the key role that L2 teachers play in designing the linguistic environments of their learning contexts and shaping learners' linguistic repertoires.