The Math Curriculum Impact Study was a large-scale randomized controlled trial (RCT) to test the efficacy of a digital core curriculum for Grade 5 mathematics. Reasoning Mind's Grade 5 Common Core Curriculum was a comprehensive, adaptive, blended learning approach that schools in the treatment group implemented for an entire school year. Schools in the control group implemented their business-as-usual mathematics curriculum. The study was completed in 46 schools throughout West Virginia, resulting in achievement data from 1,919 students. It also included exploratory investigations of teacher practice and student engagement. The main experimental finding was a null result; achievement was similar in both experimental groups. The exploratory investigations help clarify interpretation of this result. As educational leaders throughout the United States adopt digital mathematics curricula and adaptive, blended approaches, our findings provide a relevant caution. However, our findings are not generalizable to all digital offerings, and there is a continuing need for refined theory, study of implementation, and rigorous experimentation to advise schools. [For the grantee submission, see ED595114.]