The Numeracy Kit for Kindergarten 5-Year-Olds (NyKK-5) is a tool to support kindergarten teachers' numeracy practices. Based on a pedagogy of enactment, the NyKK-5 was designed to simulate the types of in-class supports that have been identified as critical elements of effective mathematics professional development (PD). Two groups of teachers received PD on children's early numeracy: The NyKK-5 condition was provided the NyKK-5 and the theory condition was not provided any pedagogical tools. Students in the NyKK-5 and theory classrooms demonstrated greater numeracy growth relative to students in a comparison condition. Teacher logs and classroom observations suggested that the amount of time spent engaging students in mathematics was more predictive of student learning than the NyKK-5 by itself, but the amount of teaching experience may define one condition under which use of the tool is optimized. [For the complete proceedings, see ED657822.]