We propose a definition of synchronous communication based on joint attention, noting that in certain mediated communication settings joint attention is a matter of perception rather than determinable fact. The most salient properties of synchronous computer-mediated communication (SCMC) are real-time pressure to communicate and a greater degree of social presence relative to asynchronous communication. These properties underlie the benefits and challenges of SCMC for language learning, which we discuss under three headings: (1) SCMC as learning tool; (2) SCMC as target competence; and (3) SCMC as setting for learner dialogue, intracultural and intercultural. We survey research themes in SCMC and preview the contributions of the Special Issue. Finally, we identify questions for future research.