Because scaffolded feedback is a key component of dynamic assessment (DA) and is present in some forms of corrective feedback (CF), it can be unclear how the 2 frameworks differ. Further complicating the distinction, many second language (L2) DA studies have focused on how a teacher provides mediation as a series of prompts, usually in reaction to a learner's error, and what that implies for L2 development. To investigate distinctions between DA and CF from an empirical stance rather than solely a theoretical one, this study utilized activity theory (AT) to analyze the changes that an English-as-a-foreign-language teacher experienced in how he responded to learners' inaccurate utterances as he endeavored to use classroom DA as part of professional development. Findings revealed key distinctions between CF and DA by showing how the discursive tools of mediation were but 1 aspect of change in the teacher's responses, and that other components of the activity system such as subject orientation, division of labor, tools beyond discourse, community, and the outcomes of the responses played a fundamental role in understanding such change. More broadly, findings also revealed how the learning of DA led to a transformation in the teacher's theoretical perspective.