Research on the outcomes of Montessori education is scarce and results are inconsistent. One possible reason for the inconsistency is variations in Montessori implementation fidelity. To test whether outcomes vary according to implementation fidelity, we examined preschool children enrolled in high fidelity classic Montessori programs, lower fidelity Montessori programs that supplemented the program with conventional school activities, and, for comparison, conventional programs. Children were tested at the start and end of the school year on a range of social and academic skills. Although they performed no better in the fall, children in Classic Montessori programs, as compared with children in Supplemented Montessori and Conventional programs, showed significantly greater school-year gains on outcome measures of executive function, reading, math, vocabulary, and social problem-solving, suggesting that high fidelity Montessori implementation is associated with better outcomes than lower fidelity Montessori programs or conventional programs. (Contains 3 tables.)