Teachers’ mathematical content knowledge and their understanding of children’s learning have been identified as critical elements in current reform movements. Hypothetical learning trajectories (Simon, 1995) have the potential to increase pre-kindergarten teachers’ understanding of the mathematical processes young children use and facilitating their students’ growth towards higher levels of mathematical knowledge. This case study of 14 pre-kindergarten teachers describes the processes these teachers utilized implementing a research- based mathematics curriculum over the course of a school year. The study found that the teachers’ reasons for volunteering influenced their use of HLT components and supports during the year. These results have implications for developing teachers’ understanding of early childhood mathematics and how reform efforts are implemented.