The goal of Project M2 was to develop and field-test challenging geometry and measurement units for K-2 students. The units were developed using recommendations from gifted, mathematics, and early childhood education. This article reports on achievement results for students in Grade 1 at 12 diverse sites in four states using the Iowa Tests of Basic Skills (ITBS) mathematics concepts subtest and an open-response assessment. Using HLM, as expected there were no statistical differences between the experimental (n = 186) and comparison (n = 174) groups on the ITBS (91% of the items focused on number). Statistically significant differences (p less than 0.001) favoring the experimental group were found on the open-response assessment with a large effect size (d = 1.88). Thus the experimental group exhibited a deeper understanding of geometry and measurement concepts on the open-response assessment while performing as well on a traditional measure that covered all Grade 1 mathematics. (Contains 7 tables and 4 figures.)