The purpose of this project evaluation was to assess the impact of data-based individualization (DBI) on the mathematics achievement of students with intensive mathematics learning needs, including students with disabilities. The evaluation study used a cluster randomized trial in which elementary schools were randomly assigned to treatment using a delayed-intervention design. Since this was a development project, the evaluation delineated between the primary, confirmatory impact question and exploratory research questions. The confirmatory question included students in Grades 1-2 and was concerned with the relationship of one year of DBI implementation support in comparison with a business-as-usual, delayed intervention group. Because of the developmental, iterative nature of the project, exploratory questions were concerned with cumulative longitudinal relations between years of DBI implementation support between two cohorts of elementary schools. In addition, project staff supported DBI implementation pilot in two middle schools and tracked student progress in those sites. Analytic results provided preliminary evidence to suggest that there may be contextual factors that govern the likelihood a student will profit from DBI. In addition, schools may require significant ongoing support to sustain implementation.