In this study, the authors examined the effects of two supplemental interventions on the mathematics achievement of low-performing intermediate-grade students. A conceptually based intervention designed to reteach fundamental mathematics was evaluated along with an intervention designed to provide extended time in the context of the core curriculum and aligned with the general education classroom's daily instruction. After a 16-week intervention with 51 low-performing fifth-grade students, the authors found that the students in both intervention groups outperformed students in the control group on a measure of mathematics achievement. On a state accountability measure in mathematics, students in the extended core group performed better than other students. Implications for instructional decision making and supplemental interventions are discussed. (Contains 2 tables.)