Research Findings: Effective preschool mathematics instruction is especially important for low-income children. Previous research demonstrates that low-income children enter kindergarten behind their middle-income peers. They receive less mathematics support at home and from public preschools. The aim of this study was to test Math Shelf, a tablet intervention designed to improve at-risk preschoolers' mathematics performance. A total of 100 children participated in a randomized controlled trial in a large urban Head Start center. Intervention students played Math Shelf on tablet computers for 6 weeks, whereas comparison students played the most downloaded and best reviewed preschool math apps on tablets for an equal amount of time. During game play, graduate student researchers supervised intervention and comparison students in separate rooms. Intervention and comparison groups did not differ on pretest assessments. Math Shelf students performed statistically significantly better (Cohen's d = 0.57) than comparison students at posttest. Practice or Policy: Math Shelf results suggest that teachers can enhance low-income preschoolers' mathematics knowledge in a relatively short amount of time by incorporating developmentally appropriate tablet interventions.