This review discusses and compares the findings of 38 peer-reviewed studies on text-based synchronous computer-mediated communication for second or foreign language learning from over the past twelve years. Research themes that emerged include: modality, corrective feedback, noticing, alignment and uptake, as well as task design/conditions and grouping. The comparison of findings revealed that the medium may enhance or hinder language learning depending on the context, method of implementation, and pedagogical aims. It also uncovers implications for language learning theory and practice that are less evident when examining each study in isolation, including findings on face-preservation and learner engagement, features of communication besides negotiation of meaning that lead to language acquisition, as well as a possible interaction between cognitive complexity and modality in the effectiveness of language learning tasks.