Teacher professional development (TPD) through supported pedagogical innovations relies on teacher understanding (TU) of what is proposed, how the innovation can be enacted, and ways in which an innovation might be effectively adapted to local conditions. This article describes evolving TU during a 1-year reading comprehension innovation. Analysis, based on a two-dimensional neo-Bloomian framework, revealed that types of TU aligned with specific strategies used at specific times in the innovation program to some extent. Initial concerns about cultural appropriateness tended to fade as participating teachers came to their own understandings of how to employ the new instructional strategies. Other teacher concerns shifted from hypothetical to more concrete concerns about how to further enhance classroom interactions, evidencing evolving TU. Importantly, while examining in-class practices shows only the outcome of an innovation, tracking teachers' developing understanding was important for gaining insights into the ongoing TPD process.