Letter-writing activities, in which pre-service teachers exchange written correspondence with K-12 students, have a rich history in mathematics education. The purpose of this research was to explore the affordances of letter-writing exchanges in the development of preservice K-8teachers’ understanding of mathematics and mathematical feedbackprocesses. Analyzing formative feedback regarding a mathematical task at the level of the task, theprocessesused to analyze the task, and the self-regulatory mechanisms involved in these processes yielded interesting results. Among these were that exposure to different ways of thinking about a task were sufficient to elicit new thinking in participants, that ametacognitive awareness of the feedback cycle could prompt change in the manner that feedback was perceived, and that improvingmathematical understanding is an incremental processthat isnot easily measured.