Background/Context: Mathematics education, especially at early grade levels, continues to be an important focus for national education improvement efforts, and more districts are introducing technology as part of their solution. Along with increasing consistent delivery of personalized and individualized learning to students, technology- and game-based programs with high-quality, engaging content offer a scalable, efficient, and effective way to help learners master targeted skills, thus supporting teachers in providing tailored instruction. Furthermore, technology- and game-based programs, with their capacity to assess and generate data on student learning, can provide information that helps teachers better understand their students' learning and plan instruction. Highly engaging educational technology innovations targeting early elementary students, such as My Math Academy, can close the learning gap and prepare students for success in mathematics. As a supplemental curriculum, My Math Academy consists of game-based activities with adaptive learning trajectories, performance dashboards that help teachers support students' learning, and offline activities that extend in-game learning experiences. Purpose/Objective/Research Question: We are conducting a large-scale randomized controlled trial to examine the efficacy of My Math Academy, a widely used, game-based, supplemental online curriculum that enables early elementary school students' personalized learning through embedded assessments, adaptive learning trajectories, and a performance dashboard that helps teachers monitor student progress and personalize learning. The study will use school-level random assignment and address the following research questions: (1) Does My Math Academy improve students' achievement in mathematics as measured at the end of kindergarten? (2) Do students who use My Math Academy during both kindergarten and first grade score higher on math achievement assessment at the end of first grade than those in the control group? (3) What are the relationships between students' My Math Academy usage, their game-based performance, and their achievement in mathematics as measured at the end of kindergarten, and then at the end of first grade? (4) What factors moderate the effects of My Math Academy? Do the effects of My Math Academy vary for students with different demographic characteristics (e.g., gender, SES, race/ethnicity, or different prior achievement levels? Setting: The study will take place in kindergarten and first grade classrooms in public elementary schools. We seek to recruit a sample of schools that includes low-performing schools and schools serving students from diverse socioeconomic backgrounds. Population/Participants/Subjects: The study sample will include 66 public elementary schools in California and Illinois, involving approximately 132 teachers and 1,980 students in kindergarten and first grade, respectively. Intervention/Program/Practice: My Math Academy is an adaptive learning system designed to help elementary age children build a strong understanding of fundamental number sense and operations. It includes 31 games with over 300 activities, covering 130 number sense and operations concepts and skills for pre-kindergarten through second grade. My Math Academy leverages games as a vehicle for playful engagement, learning in context, and formative assessments (Dohring et al., 2019). The program advocates for a mastery-based personalized learning approach (see Bingham et al., 2018; Plass & Pawar, 2020). The program integrates evidence-centered design -- an assessment framework that provides a principled alignment of the concepts, skills, and abilities a game is designed to teach with evidence of learning and task design (Clarke-Midura et al., 2012; DiCerbo et al., 2015; Shute, 2011), which enables the estimation of students' competency levels via in-game learning evidence (Mislevy, Almond, & Lukas, 2003; Shute, 2011). My Math Academy has been used by hundreds of thousands of children and several thousand teachers since 2018. Self-funded pilot studies have demonstrated the promise of the program (Bang & Li, 2020; Thai, Li, & Schachner, 2018). Using My Math Academy, students will work through personalized learning pathways with interactive, story-based learning activities that are customized to individual needs. They will receive instruction on math concepts and skills, scaffolding, and feedback to help solve difficult problems, and embedded assessments to gauge their understanding of individual concepts and skills. Teachers will receive real-time reports to facilitate their review of students' usage and progress and adapt instruction accordingly. The theory of change (Figure 1) shows how the theoretically grounded components lead to changes in math achievement. Research Design: The study will use a clustered randomized experimental design. Schools will be randomly assigned to either the treatment or control condition and will be asked to participate for two full school years (kindergarten: 2023-2024 and first grade: 2024-2025). Treatment teachers and students in kindergarten and first grade will use My Math Academy as a supplemental curriculum, three time per week. Teachers will receive professional development before they start implementing My Math Academy and use it along with the accompanying student data as a tool to personalize classroom practices. In the control condition, teachers will continue their existing instructional practices and supplemental technologies. Data Collection and Analysis: Data on implementation fidelity and contrast between conditions will be collected during both years. Cost data will be gathered systematically throughout the study. The Test of Early Mathematics Ability, third edition (TEMA) will be the primary measure of student early mathematics skills. Efficacy will be examined using a three-level hierarchical linear regression model of mean differences in TEMA scores between students in two conditions, controlling for prior achievement and other covariates. Moderator analyses will examine the impact of the intervention on the learning of students with low-baseline mathematics achievement and different demographic backgrounds. Mediator analyses will examine the link between student learning outcomes and mediating variables such as engagement in mathematics learning. Exploratory analyses will address the accumulated impact of multi-year My Math Academy exposure on student outcomes. Analysis will also be conducted to examine cost and cost-effectiveness of My Math Academy.