This chapter reports research which examines the processes of, and relationship between, learner-learner interaction and knowledge construction within an online undergraduate communication course. Vygotsky’s theory of development serves as a sensitising framework for the investigation. The research strategy is a single case study with an embedded case design. Social network analysis and constant comparative method are utilised to analyse data collected from the case. The results reveal how a single cohort of learners interacts and construct knowledge within large and small groups using asynchronous and synchronous communication. A substantive theory, which explains the conditions, interactions and consequences of learning relationships in online contexts, has been constructed. The research has important implications for educational practice as it reveals a range of conditions which are conducive for learner-learner interaction, dialogic learning and a sense of community in online courses.