Understanding of base 10 and place value are important foundational math concepts that are associated with higher use of decomposition strategies and higher accuracy on addition problems (Laski, Ermakova, & Vasilyeva, 2014; Fuson, 1990; Fuson & Briars, 1990; National Research Council, 2001). The current study examined base-10 knowledge, place value, and arithmetic accuracy and strategy use among children in early elementary school from Montessori and non-Montessori schools. Children (N = 150) were initially tested in either kindergarten or first grade. We followed up with a subgroup of the sample (n = 53) two years later, when the children were in second and third grades. Although Montessori curriculum puts a large emphasis on the base-10 structure of number, we found that children from Montessori schools showed an advantage on correct use of base-10 canonical representation in kindergarten but not in first grade. Moreover, no program differences were seen in place-value understanding in second and third grades. Although Montessori children used different strategies to obtain answers to addition problems in second and third grades as compared with non-Montessori children, no program differences in addition accuracy were found at any grade level. Educational implications are discussed.