This research responds to the call by early childhood educators advocating for more challenging mathematics curriculum at the primary level. The kindergarten Project M² units focus on challenging geometry and measurement concepts by positioning students as practicing mathematicians. The research reported herein highlights the features of the kindergarten units and reports on student achievement, including the differences between the intervention and comparison groups. Hierarchical linear modeling results indicate that students in the intervention group (n = 210) outperformed those in the comparison group (n = 196) on the Iowa Tests of Basic Skills Mathematics subscale (p < .01; d = 0.25), and an open-response assessment (p < .001; d = 2.68) encompassing performance-based, verbal, and written components. These results suggest that kindergarteners can achieve at high levels of mathematical understanding when given the opportunity to learn from more challenging curriculum.