Three decades of research have established that simultaneous computer-mediated communication (SCMC) can impact L2 development processes, including corrective feedback, and their products, in new and exciting ways. However, the high variability of the evidence calls for more theoretical and methodological rigor, and the proliferation of hybrid and online learning curricula makes it imperative to assess SCMC’s contributions on the basis of sound comparisons with mirror FTF conditions. Against this backdrop, this chapter critically synthesizes forty-one studies comparing the effects of written SCMC versus FTF interaction on L2 development, filtering contradictory findings through the sieve of methodological validity and the often-neglected notion of modality. Results suggest that written SCMC amplifies feedback effectiveness in tutor–learner interactions but not so clearly in peer interactions; that it promotes lower output rates but more equal participation; that it elicits shorter utterances, but maybe more accurate and complex grammar and vocabulary; that it promotes less negotiation for meaning but maybe denser and more explicit negotiation of form; that it is more likely to subvert the negotiate-over-lexis-first principle; and that it yields less successful uptake and maybe less modified output but more self-corrections. The chapter concludes with future research recommendations based on the three dimensions of modality.