The authors conducted a small-scale randomized control trial (n = 31 teachers) of Online Content-Focused Coaching, an intervention consisting of an online workshop followed by multiple cycles of remote video-based coaching, to support dialogic text discussions. Findings demonstrate the efficacy of Online Content-Focused Coaching in three different ways. First, the authors' analyses, after accounting for differential attrition among groups, demonstrate an existence proof for effects of the intervention on both classroom text discussion quality and student achievement. Second, the authors examined and demonstrated an association between the magnitude of changes in discussion quality and the magnitude of achievement gains. Finally, the authors propose and examine evidence to support a theory for how teachers develop adaptive expertise for facilitating dialogic text discussions. Results show that teachers' use of transitional and some aspirational discussion moves grew from baseline to the end of the workshop, with limited growth in the quality of students' contributions. Over the coaching phase of the intervention, teachers' facilitation moves grew substantively, and so did students' strong contributions. The authors interpret the results to suggest that the workshop was critical for developing teachers' knowledge of the features of dialogism and that coach-guided reflection was essential for developing teachers' expertise at using facilitation moves to elicit student thinking. Findings contribute to a validity argument for the efficacy of Online Content-Focused Coaching. More importantly, investigating and describing the process of teaching change is the study's main theoretical contribution.