ASSISTments is a web-based math tutor designed to address the need for timely student assessment while simultaneously providing instruction, thereby avoiding lost instruction time that typically occurs during assessment. This article presents a quasi-experiment that evaluates whether ASSISTments use has an effect on improving middle school students' year-end test scores. The data was collected from 1240 seventh graders in three treatment schools and one comparison school. Post-test (7th grade year-end test) results indicate, after adjusting for the pre-test (6th grade year-end test), that students in the treatment schools significantly outperformed students in the comparison school and the difference was especially present for special education students. A usage analysis reveals that greater student use of ASSISTments is associated with greater learning consistent with the hypothesis that it is useful as a tutoring system. We also found evidence consistent with the hypothesis that teachers adapt their whole class instruction based on overall student performance in ASSISTments. Namely, increased teacher use (i.e., having more students use the system more often) is associated with greater learning among students with little or no use, suggesting that those students may have benefited from teachers adapting their whole-class instruction based on what they learned from ASSISTments use reports. These results indicate potential for using technology to provide students instruction during assessment and to give teachers fast and continuous feedback on student progress. (Contains 4 figures, 1 table, and 3 footnotes.)