We examined a web-based mathematics progress monitoring tool for second graders. The tool monitors the learning progress of two competences, number sense and computation. A total of 414 students from 19 classrooms in Germany were checked every 3 weeks from fall to spring. Correlational analyses indicate that alternate-form reliability was adequate for the chosen interval (0.81 < r < 0.87). Results from latent growth curve modeling (LGCM) identified significant linear increases in students' scores, and a significant variance in computation slopes was observed. Criterion validity coefficients, comparing the measures with student performance on standardized school achievement tests at the beginning and the end of the school year (DEMAT1+, DEMAT2+), were satisfactory (0.59 < r < 0.76). Students conducted the tests independently, and assessment times were short. Implications for further research and classroom practice are discussed.